Instructing experts and hindsight
Approximation of skilled person
Points of foreign law
CGK and skilled team
Other practice points



The conclusion of Mrs Justice Joanna Smith, presiding in the Technology and Construction Court in 2021, that "the use of experts only works when everyone plays by the same rules"(1) should resonate deeply with patent litigators in the United Kingdom, for whom the strength of the evidence of an expert witness can make, or break, a case.

8 February 2022 will mark one year since the appointment of Mr Justice James Mellor to the High Court of Justice of England and Wales. He joined fellow patents specialist judge, Mr Justice Richard Meade, appointed five months prior. In their short tenure thus far, both judges have frequently commented on the value of the expert evidence before them, and in doing so have provided further helpful guidance for the instruction of experts in almost all of the patents judgments they have handed down. This article provides a brief overview of some of the key takeaways patent litigators can take from these recent decisions.(2)

Instructing experts and hindsight

In patent litigation proceedings, the primary function of an expert is to educate the court.(3) Solicitors instructing experts must ensure that the expert is "put in a position to express an independent and impartial position",(4) ensuring that they understand that their overriding duty is to the court, not to the party instructing them.(5) In the context of validity, particularly obviousness, it is crucial that the expert's view is free from the influence of hindsight – an expert must express what was common general knowledge (CGK) at the priority date of the patent in suit and give their views on any cited prior art without having regard to the claimed invention.

So as to minimise the risk of hindsight creeping into expert evidence, the preferred approach by the Patents Court, particularly advocated by Mr Justice Richard Arnold (now Lord Justice Arnold) has been for experts to discuss the CGK with their instructing solicitors first, before then being shown relevant prior art and, only afterwards, reviewing the patent in suit.(6)

This "sequential unmasking" approach continues to be favoured by Meade J, who held in Fisher & Paykell Healthcare v Flexicare(7) – one of his first patent decisions as a High Court judge – that instructing experts in this way remains preferable, albeit not mandatory. However, practitioners will know that this can be tricky, as Meade J has acknowledged. In both Fisher & Paykell and his later judgment in Promptu v Sky,(8) he emphasised that, in certain scenarios, the sequential instruction of experts may not be practicable or even possible, such as where the expert already knows of the invention, or where further expert evidence needs to be submitted at a later date to address points not made in a first report.

Indeed, the role of hindsight often presents a significant challenge in instructing experts. Both Fisher & Paykell and Promptu also reinforce the idea that an expert cannot be accused of relying on hindsight simply because instructing solicitors have directed them to particular parts of the prior art or have given them guidance on specific aspects of the CGK to explain. Meade J's pragmatic view is that asking open-ended questions to an expert, such as "what was the [CGK] in your field?",(9) or asking an expert to review lots of passages in a prior art document (which may be irrelevant) would lead to a task that is "too big and too diffuse".(10) If instructing in this way would lead to the risk of hindsight, the court may have to take this into account, but – provided the expert "acknowledges the points and gives evidence about why that part of the art would be of interest"(11) – instruction in this way is not necessarily fatal or even serious to a case.

This is welcome relief for practitioners who may be unable, in the short time available with busy experts, to have them undertake a detailed review of all facets of a prior art document, even though these are plainly irrelevant to the case at hand. This approach must evidently be taken with some caution, however; as stated by Arnold J (as he then was) in MedImmune, where an expert is silent on any passages in their expert report, this leaves them open to questions under cross-examination as to why they did not mention such passages, and accusations that they have failed in their overriding duty to the court.

Meade J provided further guidance for practitioners with regard to hindsight in Philip Morris v BAT,(12) stating that there is:

a difference between asking an expert witness the open question 'what would the skilled team do?' and, for example, the question 'would it be obvious to use [a feature of the claimed invention]?'. The latter bears the risk of introducing hindsight.

Not to be left out of the discussion on hindsight, Mellor J in Teva v Bayer Healthcare(13) has reiterated that regard should be had to the issue of hindsight both by the party challenging the validity of a patent and the patentee:

Hindsight can infect the response to an obviousness case if, for example, a witness (knowing of the 'target') appears to be looking for ways to avoid taking a particular step or making a particular choice towards the target, when the Court assesses that the Skilled Person would consider the step or choice differently and, having considered it dispassionately, would decide to take the step or choice.(14) (Emphasis added.)

In this regard, Mellor J found that the patentee's expert had placed too great an emphasis on theoretical points, founded on hindsight and detached from the real-world approach of the skilled team, which would have been to undertake practical tests. Meade J similarly viewed evidence less favourably in Optis v Apple,(15) where the defendant's expert had concerned himself more with an abstract, theoretical approach and not with what approach would have been taken in reality. The comments from both judges come back to the overriding objective of an expert to assist the court and not to act as an advocate for either side.

Approximation of skilled person

It is common for parties to patent proceedings to challenge the suitability of the other side's witness to stand as an expert in the proceedings. However, this particular point has been given relatively little weight by Meade J in the vast majority of his patent decisions over the last 18 months. He has reiterated time and again the principle that "it is well established that the assessment of expert evidence in patent cases is not an exercise in determining which expert in fact approximates more closely to the notional skilled person".(16)

The fact that an expert may have a number of patents to their name,(17) and, thus, potentially be more innovative than the skilled person or, at the opposite end of the spectrum, have less direct experience in a particular field(18) should be of less importance, provided the expert is able to put themselves in the shoes of the ordinary skilled person at the priority date. To that extent, an expert with less direct experience should be able to achieve this by drawing on any experiences pre- and post-priority and by reading relevant literature.(19) This again should be seen as welcome respite for practitioners seeking to find a suitable expert where the field of interest is narrow.

By contrast, where less weight has been given recently to the evidence of an expert, it is generally where that individual has less real-world experience to draw upon.(20)

Points of foreign law

The role of evidence and experts on aspects of foreign law provides additional considerations. In Optis, Meade J was asked at the pre-trial review (PTR) to make an order that there be no oral evidence on a particular point of French law, and, therefore, no cross-examination. As he noted, this is a relatively common approach in IP cases "where it is anticipated that the cross-examination would consist merely of putting to the experts written materials which the Court could just read itself". The parties are expected to point out the key foreign law materials and make arguments on them such that the court would be in the same position as if there were cross-examination.

Clearly, this approach has the benefit of saving costs and court time, and Meade J criticised the parties in Optis for failing to communicate the key issues, holding that they "should have considered and discussed what was in issue so that [he] did not have to spend time reading a lot of French evidence and submissions that went to nothing". It is possible, should similar issues of foreign law arise in the future, that more active management can be expected from the court in this regard. Meade J stated that he wished he had "managed matters more proactively at the PTR, and in particular should have thought about directing a meeting of the French experts and an agreed joint memorandum".

CGK and skilled team

A key question for expert evidence in patents cases is what amounts to CGK. In Alcon, the claimants attacked the route by which the defendants' witnesses had arrived at certain documents demonstrating the CGK at the time. In this regard, Meade J held that "it is not necessary or realistic to prove exactly by what route the skilled addressee would find or access CGK, and it may well be enough to show that the information is contained in a number of well-known reference-type works", even where such works had been provided to the expert by the instructing solicitors (as in Alcon). This again appears to be a pragmatic approach by Meade J to the instruction of experts, where the date for determining the CGK may be over 20 years in the past.

Linked to this, an issue that has been discussed by Meade J in both Fisher & Paykell and Alcon, is the need to ensure that the person skilled in the art or the skilled team is defined with sufficient specificity such that the CGK is similarly appropriately framed. To define the skilled team too narrowly runs the risk of imputing knowledge to the skilled team that should not rightly be deemed CGK, thus harming the patentee. By contrast, defining the skilled team too broadly may cause harm to the defendant (and, thus, the public) by diluting the knowledge of the skilled team in a particular area, therefore impacting on the likelihood that the patent will be deemed obvious.

Other practice points

There a few further practice points that have been reiterated in the recent decisions of Meade J and Mellor J with respect to the instruction of experts and that should also be borne in mind by patent litigators.

It is common, and perhaps almost expected, for an expert to be criticised to varying degrees by opposing counsel. However, such criticism should of course be measured, and justifiable by counsel, notwithstanding the judge's own views of the expert and their evidence. Along these lines, in Mitsubishi v Oneplus,(21) Mellor J termed the cross-examination of one expert as "unnecessarily aggressive", noting that counsel had responded to an answer given by the expert by saying "I cannot believe that really is your evidence".

Expert evidence should, of course, be supported. In their expert reports, an expert should always support their opinions with citations. In MSD, Meade J identified an expert's failure to do so as a real a point of concern, noting that the expert in question had advanced "speculative ideas", which he "should not have advanced without some proper support". This segues nicely on to Meade J's comments in Fisher & Paykell that "what really matters with expert witnesses in patent cases is not their conclusions but their reasons". In that case, a failure to do so reinforced the judge's concerns about the expert in question.


The judgments of Justices Mellor and Meade over the last 12 to 18 months serve as a useful reminder of some of the key principles relating to the instruction of experts in patent litigation proceedings in the United Kingdom that the court has built up over recent years. The prevalence of these comments in the relatively small number of patent judgments from both judges since their appointment to the High Court suggests that maintaining a well-defined framework in which experts should operate will no doubt be a continuing point of interest for future cases. A failure for patent litigation practitioners to follow this guidance could lead to cost consequences, an undermining of the credibility of their clients' evidence or, ultimately, such evidence being dismissed altogether.

For further information on this topic please contact Charlie Davies at Pinsent Masons by telephone (+44 20 7418 8250) or email ([email protected]). The Pinsent Masons website can be accessed at


(1) Dana UK Axle v Freudenberg FST [2021] EWHC 1413 (TCC) at [94].

(2) The decisions handed down by Meade and Mellor JJ when sitting as a Deputy Judges of the High Court are not the focus of this article.

(3) Technip France SA's Patent ([2004] RPC 46.

(4) Medimmune v Novartis ([2011] EWHC 1669 (Pat).

(5) Civil Procedure Rules, Part 35.3(1).

(6) MedImmune.

(7) [2020] EWHC 3282 (Pat).

(8) [2021] EWHC 2021 (Pat).

(9) Fisher & Paykell at [22].

(10) Promptu at [15].

(11) Ibid.

(12) [2021] EWHC 537 (Pat).

(13) [2021] EWHC 2690 (Pat).

(14) Teva v Bayer at [113].

(15) [2021] EWHC 1739 (Pat).

(16) Philip Morris.

(17) Add2 v Dspace [2021] EWHC 1630 (Pat).

(18) Merck Sharp and Dohme (MSD) v Wyeth [2020] EWHC 2636 (Pat).

(19) MSD.

(20) See Meade J in Add2 and Alcon v Aspire [2021] EWHC 1026 (Pat).

(21) [2021] EWHC 1639 (Pat).