In a recent case concerning the wonderfully named Pepe's Piri Piri Ltd, an expert got his fingers burned. The judge criticised him for failing to understand his duties and comply with them.
The rules are clear. An expert's duty is to the Court. The duty of experts is "to help the court on matters within their expertise" . That duty "overrides any obligation to the person from whom experts have received instructions or by whom they are paid" .
An expert may want to help the party instructing him or her – perhaps out of sympathy for their case. They may have an unconscious bias tending to support that case or maintain an opinion for which the evidence is becoming less favourable. But their overriding duty is to the Court.
An accountant giving expert evidence in the Piri Piri case felt the heat when the judge criticised him at trial for failing to fulfil that duty.
The criticisms were:
When asked in cross-examination about his duty, he said that although his ultimate duty was to the Court, where he was instructed by a particular party, he would do his best to put that party's case in the most favourable light. That was inconsistent with his duties under Civil Procedure Rules Part 35. The judge's words bear considering – they are clear comments on the duties of experts to maintain their independence:
"It is not part of the duty of an expert to advance the case of the party instructing them, whether by advancing arguments of fact or law which are outside their expertise or by seeking to present that party's case in a favourable light. An expert witness should present evidence which is uninfluenced by the pressures of litigation and contains independent assistance by way of objective opinion."
That did not mean that he entirely rejected the expert witness's evidence. But where there was a conflict between the two experts, the judge placed greater weight on the claimant's expert.
Unfortunately, getting it wrong in a misguided attempt to promote one party's case may well undermine your evidence, as it did in this case. In short, understand your duties and don't get your fingers burned.
You can read more on this topic in Paul Sankey's article, '7 classic principles for expert witnesses'.
 Pepe's Piri Piri Ltd & Anor v Junaid & Ors  EWHC 2097
 CPR 35.3(1)
 CPR 35.3(2)
 PD 35.2.1
 PD 35.2.2
 PD 35.2.3
 PD 35.2.4(a)