FREE CHAPTER from ‘A Practical Guide to the Use of Expert Evidence in Criminal Cases – Second Edition’ by Richard Padley


” ….It is true that the witness who is called upon to give evidence founded on a comparison of handwritings must be peritus ; he must be skilled in doing so; but we cannot say that he must have become peritus in the way of his business or in any definite way. The question is, is he peritus? Is he skilled? Has he an adequate knowledge?.”1

Over 125 years since the above comment was made in Silverlock expert evidence, its quality, its appropriateness, and its limitations remain of great debate within the criminal jurisdiction. The past few years have seen high profile cases highlight that not all individuals advanced as experts are necessarily as ‘peritus’ as they believe themselves to be. Time used scrutinising and examining an expert’s report, their background and qualification, can prove more fruitful than one may originally consider.

What is expert evidence?

Expert evidence is evidence of opinion falling outside the experience of a judge or jury. By contrast non-expert evidence goes to matters of fact. The oft quoted judgement of King CJ in the South Australian case of R v Bonython notes the distinction:

whether the subject matter of the opinion is such that a person without instruction or experience in the area of knowledge or human experience would be able to form a sound judgment on the matter without the assistance of witnesses possessing special knowledge or experience in the area; and whether the subject matter of the opinion forms part of a body of knowledge or experience which is sufficiently organised or recognised to be accepted as a reliable body of such and a special acquaintance with it by the witness would render his opinion of assistance to the court.”2

The Court of Appeal observed in R v Dlugosz and others3 that in considering the issue of whether the evidence sought to be adduced is expert evidence, and potentially admissible as such, the court must be satisfied that there is a sufficiently reliable scientific basis for the evidence to be admitted and tested before the jury. The untested views of an individual are unlikely to find favour before the criminal courts.

Areas of expertise that practitioners will have often come across involve elements of technical or forensic analysis. These can include:

  • DNA evidence – to assess the likelihood that DNA found on an item of interest has come from a person of interest

  • Psychiatric evidence – to assist establish an individual’s mental state at a particular time of relevance

  • Firearms evidence – in assessing whether a particular item falls within the definitions under the Firearms Act 1968

  • Cell site evidence – to establish location and movement of a sim card

  • Facial recognition – to assess the likelihood that a person shown on CCTV/photograph is the same as a person of interest

However expert evidence is not exclusively based around science or forensic analysis. Areas of expertise perhaps less routinely used have included:

  • Historical facts about Al-Qaeda – Ahmed [2011] EWCA Crim 184

  • The political and economic situation in Russia – Berezovsky v Hine [2011] EWHC 1776 (Ch)

There are however areas where an expert should not stray. It is now well established that an expert cannot go to the question of whether or not an individual is truthful.4 The question of whether a witness is truthful and reliable is a question for the trier of fact and not for the expert.

Additionally, whilst matters of foreign law can often be subject to expert evidence the law of England and Wales is not a proper subject for expert evidence since it falls within the competence of the court.

The evidence an expert gives should be based on admissible evidence, and it is incumbent on the advocate calling the expert to adduce the underlying admissible facts during the evidence.5

Who can be an expert?

An expert is an individual who has specialist knowledge in a particular subject area. That specialist knowledge can be gained through years of experience, whether in the pursuit of research and academic study, the practical application of the field of knowledge through work, or often a combination of both.

52. The next criterion is whether relevant expertise had been demonstrated. The old authorities talk of ‘men of science’ and much of the discussion about expertise revolves around recognised disciplines in science or medicine. Yet it is well recognised that it is possible to become an expert by practice and experience6.

As outlined in the above passage from R v Brecani, qualification as an expert can be gained through academic qualification or vocational practice and experience. Some areas of expertise are constantly changing and being updated, and consequently ‘on the job’ experience is essential. Very often experts will have a combination of both academic qualification and practical experience form which they can draw.

In selecting an appropriate expert, the interplay between their knowledge and expertise is a relevant consideration. Whist an expert is entitled to apply their knowledge to reach a conclusion by, for example, referring the academic papers of others in the field7, that knowledge should be evaluated by the expert by reference to their practical day to day experience8. An individual who has a purely academic knowledge of a subject may certainly be described as an expert, but the opinion given by such an expert may well be given less weight with the court than that of a person with a combination of academic knowledge and experience9. That is not to say that academic training cannot engender appropriate experience in the right circumstances.

The importance of qualification was explored in the case of Brecani10 when considering the admissibility of the ‘conclusive grounds’ decision in the assessment of whether an individual could be categorised as a ‘modern day slave’. The Court of Appeal noted the following as part of their decision in ruling the evidence inadmissible:

53. In this case, and in respect of decisions by case workers assigned to the Competent Authority generally, no information was available upon which it is possible to determine whether and in what area the case worker might be an expert. Beyond an inference that decision making is approached via the comprehensive guidance to which we have referred and that there must be some training provided, case workers in the Competent Authority are junior civil servants performing an administrative function which includes making reasonable grounds and conclusive grounds decisions. The guidance recognises that a decision of the Competent Authority might be disclosed in criminal proceedings but nowhere is there any suggestion that the decision maker might be called to give evidence, still less that in making the decision the case worker is acting as an expert and might give evidence in chief and be cross-examined in a criminal court as such.

54. In respectful disagreement with the Divisional Court in DPP v M we do not consider that case workers in the Competent Authority are experts in human trafficking or modern slavery (whether generally or in respect of specified countries) and for that fundamental reason cannot give opinion evidence in a trial on the question whether an individual was trafficked or exploited. It is not sufficient to assume that because administrators are likely to gain experience in the type of decision-making, they routinely undertake that, simply by virtue of that fact, they can be treated as experts in criminal proceedings. The position of these decision-makers is far removed, for example, from experts who produce reports into air crashes for the Air Accident Investigation Branch of the Department of Transport which are admissible in evidence in civil proceedings: see Rogers v Hoyle [2015] 1 QB 265 . Moreover, none of the requirements of CrimPR 19, designed in part to ensure that the person giving evidence is an expert, understands he or she is acting as such and understands the obligations of an expert to the court, were complied with.

In the above passage the Court of Appeal makes a number of important points. First, the requirement to comply with part 19 of the Criminal Procedure Rules. As set out in Chapter 2 Part 19 of the Criminal Procedure Rules provide the relevant provisions relating to expert evidence. Secondly, repetition of process does not make an expert. As was outlined in Brecani decision makers under the National Referral Mechanism follow guidance and perform an ‘administrative function’. The act of performing an administrative role in line with guidance provided was insufficient to make an individual an expert in the area. But the Court did leave open the possibility to the admissibility of expert evidence in the area of modern slavery. The Court commented:

58. Mr Douglas-Jones appeared at the appellate level for the prosecution in DPP v M . At [45] the court noted that it was no part of his argument that expert evidence was never admissible on the question of trafficking and exploitation. He readily accepted before us that there can be circumstances in which a suitably qualified expert might be able to give evidence relevant to the questions that arise under the 2015 Act which are outside the knowledge of the jury, particularly to provide context of a cultural nature. That is clearly correct. The evidence would have to be truly expert and not a vehicle to enable the expert to stray into the territory of the jury by expressing his or her personal opinion about whether an account is credible or inconsistencies immaterial. As the present case vividly illustrates, the conclusion on whether the prosecution has disproved the section 45 defence will call for an assessment of all the relevant evidence which the jury is well-placed to make. In the right case that evidence might include expert evidence of societal and contextual factors outside the ordinary experience of the jury.

The developing area of the use of modern slavery experts is explored further at Chapter 10.

Challenging Expertise

Whilst many profess to have expertise, whether an individual truly does, and whether they hold said expertise in the area relevant to the matters in issue. It is an important principle that an expert can only give expert evidence in an area within their expertise. The case of R v Alex Pabon11 started a string of cases in which this issue was brought to the fore.

Alex Pabon

Alex Pabon was found guilty after trial of a single count of conspiracy to defraud, in respect of dishonestly rigging the London Interbank Offered Rate (LIBOR). During the trial the SFO relied upon an expert witness, Saul Rowe. Mr Rowe was also used in the retrial of two of Mr Pabon’s co-defendants, Mr Stylianos Contogoulas and Mr Ryan Reich. During the retrial material was revealed to the prosecution and the defence undermining Mr Rowe’s abilities to act as an expert in these proceedings – email correspondence was disclosed that another partner at Mr Rowe’s firm had in fact produced parts of Mr Rowe’s expert report, and text messages from Mr Rowe to others relating to matters on which he was giving evidence.

In summing up the matter to the jury HHJ Leonard QC highlighted the matter within the following passage:

“Despite that catalogue of experience, you may have formed a judgment that he knew very little about the duties of being an expert……he seems to have been perfectly content to sign a standard declaration in which he declared that he had read the Criminal Procedure Rules which govern his conduct as an expert, both before trial and in giving evidence, and the booklet on his duties of disclosure without doing anything really to familiarise himself with either of those documents. It will be for you to judge whether he has in fact given expert opinion which falls outside his true expertise. Any expert is entitled to research a topic on which he is to give evidence and obtain the views of others, including work colleagues, about it to enhance his opinion, so long as he records where he went for that advice and so long as it is to enhance an expertise he already has, rather than to become an expert on a subject where he has no knowledge whatsoever.12

The passage highlights several important matters:

1. It is for the jury to determine, upon hearing the evidence, how much weight to put on an expert’s evidence.

2. Whilst an expert is entitled to update their knowledge of matters on which they are due to give evidence including by approaching third parties, those communications should be logged and reviewed for disclosure.

Evidently, and perhaps without needing to be stated, it is clearly undesirable for an expert witness to attempt to become an expert in an area they hold no previous expertise during the trial.

In considering what makes an expert the court went back to 1894 and the case of Silverlock13 and the judgment of Chief Justice Killowen:

” ….It is true that the witness who is called upon to give evidence founded on a comparison of handwritings must be peritus ; he must be skilled in doing so; but we cannot say that he must have become peritus in the way of his business or in any definite way. The question is, is he peritus? Is he skilled? Has he an adequate knowledge? Looking at the matter practically, if a witness is not skilled the judge will tell the jury to disregard his evidence. There is no decision which requires that the evidence of a man who is skilled in comparing handwriting, and who has formed a reliable opinion from past experience, should be excluded because his experience has not been gained in the way of his business……”

As explored earlier in this chapter, and highlighted in the above passage, an individual can become an expert in a broad range of ways. As was noted by Bingham LJ in R v Robb14 English law is “characteristically pragmatic”15 in establishing expertise. This benefit of this broad approach to characterising expertise allows the issue to be examined on a case-by-case basis taking into account the particular individual advanced as an expert and the particular issues in the case that are highlighted as requiring expert evidence.

Ultimately the Court of Appeal upheld the convictions in Pabon and others based on the relevance of the expert’s evidence to the ultimate matter in issue at the trial – the expert didn’t speak to the issue of dishonesty in respect of the Appellants’ actions, and therefore could not have affected the safety of the convictions.

James Byrne and others [2021] EWCA Crim 107

Almost as soon as the Pabon case was ending issues surrounding a different expert witness were starting to arise. Between 2016 and 2019 a series of individuals were convicted of dishonestly engaging in selling carbon credits. A common factor in these cases was the use of a consistent expert witness, Andrew Ager.

In May 2019 Mr Ager was called upon again to give expert evidence in Operation Balaban heard at Southwark Crown Court. During the course of a voir dire Ager was discredited and abandoned as a witness by the prosecution on the grounds that it was accepted that he was not an expert of suitable calibre. During Mr Ager’s cross examination in the voir dire it was established that he had received no training in the duties of being an expert, considered it was appropriate to withhold important concessions from the defence, held no relevant academic qualifications, made no attempt to comply with other obligations of an expert witness including failing to retain and reveal records and material, including material relevant to meetings he had attended16.

In discharging the jury in the Operation Balaban trial the Judge drew from Lord Justice Goss’ comments in Alex Pabon: ‘This case stands as a stark reminder of the need for those instructing expert witnesses to satisfy themselves as to a witness’ expertise and to engage (difficult though it sometimes may be) an expert of a suitable calibre’. The Judge commented that time was to show that Andrew Ager was not an expert of a suitable calibre.

What followed was a series of appeals, conjoined due to a common theme, by Appellants who had all been convicted following trials in which Andrew Ager had given evidence. The Appellant’s primary ground was that Mr Ager’s failures highlighted in Operation Balaban undermined his evidence in their respective trials and therefore made their own convictions unsafe.

The Court of Appeal dismissed all appeals. They concluded that although Ager’s lack of formal qualification should have been made clear it was not, by itself, determinative of whether he was entitled to give expert evidence. It was noted that Ager had some considerable experience in the financial commodity and carbon credits market. Further, as with the approach taken in Pabon, the court sought to consider how Mr Ager’s failings in 2019 fit in with the evidence as a whole in relation to the convictions being appealed, coming back to the test of whether the convictions were consequently unsafe. It noted that there was no dispute to Ager’s evidence in the other trials. There was no counter expert evidence (unlike in Balaban). The issue in the appellants’ trials was participation. The Court reiterated that Ager’s evidence was unchallenged. Ultimately the appeals were dismissed. It was considered untenable to conclude that the present convictions were unsafe because a witness in later proceedings could be demonstrated to have acted unprofessionally in the context of that case.

The independence of experts

Criminal Procedure Rule 19.2 outlines that the expert’s duty lies to the court rather than the party who has instructed them.

Expert’s duty to the court


(1) An expert must help the court to achieve the overriding objective—

(a) by giving opinion which is— (i) objective and unbiased, and (ii) within the expert’s area or areas of expertise; and

(b) by actively assisting the court in fulfilling its duty of case management under rule 3.2, in particular by— (i) complying with directions made by the court, and (ii) at once informing the court of any significant failure (by the expert or another) to take any step required by such a direction.

(2) This duty overrides any obligation to the person from whom the expert receives instructions or by whom the expert is paid.

This overriding duty will often manifest itself in the expert’s report in the form of a declaration confirming that the expert understands their duty is to help the court, by giving assistance by way of objective and unbiased opinion on matters within the expert’s expertise, and that duty overrides any obligation towards the party by whom they are engaged or paid by.

It is well established that investigating officers can provide expert evidence17. In the conjoined appeals of Myers, Cox and Brangman v R18 in which the Privy Council heard an appeal from the Court of Appeal for Bermuda Hughes SJC noted the following:

But the officer must have made a sufficient study, whether by formal training or through practical experience, to assemble what can properly be regarded as a balanced body of specialised knowledge which would not be available to the tribunal of fact. … But care must be taken that simple, and not necessarily balanced, anecdotal experience is not permitted to assume the robe of expertise. An example is given by the Canadian case Sekhon v The Queen [2014] 1 SCR 272 , where in the course of otherwise perfectly permissible expert evidence as to drug practices, there was adduced from the officer the fact that he had never encountered an innocent courier. That was clearly not a balanced, tested, or researched proposition as to the methods of drug importers, but simply his personal experience. It was not admissible and indeed proved nothing about the particular defendant on trial. Secondly, whilst a police officer may be an expert, by training or experience or both, if he is then he comes under the same duties to the court as does any other expert.19

The court went on to note that compliance with the requirements of an expert may be difficult for a police officer who is combining the duties of an active investigator with providing independent expert evidence. Police witnesses who become expert witnesses should understand they are not part of the prosecution team but have separate independent duties to the court.

Police officers are entitled to give expert evidence on several different topics:

  • Quality of driving – Oakley20 – an officer with qualifications and experience in accident investigation can give evidence of how an accident occurred.

  • Supply of drugs – R v Hodges and Walker21 [2003] – an officer’s evidence was capable of challenge during cross examination and by any evidence to the contrary adduced by the Defence.

  • Gang related evidence – see R v Dixon-Kenton [2021] EWCA Crim 673 where the Court of Appeal observed that expert evidence about the existence of gangs, their ways of operating, their language and culture is increasingly adduced in the courts. It was noted that police officers in several cities have developed expertise in the gang culture in the areas in which the officers operate. Referring to Myers it was noted that the admissibility of such evidence is well recognised.

Whilst a conflict of interest does not automatically disqualify the individual from giving expert evidence, where it is material and significant the court is unlikely to grant permission for the evidence to be called. Inevitably, any conflicts of interest must be disclosed so the matter can be addressed by either side.


1[1894] 2 QB 766 at 771

2R v Bonython [1984] SASR 45

3[2013] EWCA Crim 2

4R v C [2012] EWCA Crim 1478

5R v Terence Turner [1975] QB 834

6R v Brecani [2021] EWCA Crim 731 at para 52

7R v Abadom [1983] 1 WLR 126

8R v Weller [2010] EWCA Crim 1085 at para 38


10[2021] EWCA Crim 731

11[2018] EWCA Crim 420

12As outlined in R v Pabon [2018] EWCA Crim 420 at para 52

13[1894] 2 QB 766 at 771

14[1991] 93 Cr App R 161

15Ibid at 164

16[2021] EWCA Crim 107 at para 15

17See R v Oakley [1979] RTR 417 – “we would like to make it quite clear straight away that there is no question of a police officer being prevented from giving evidence as an expert if the subject in which he is giving evidence as an expert is a subject in which he has expert knowledge, and if it is restricted and directed to the issues in the case.”

18[2016] AC 314

19Ibid at para 58-59

20[1979] 70 Cr App R 7

21[2003] EWCA Crim 290