In legal cases, this issue of advocacy usually arises where an expert uses language that appears to be advocating one party's case rather than giving independent evidence to assist the Court. However, in a recent tax case, the advocate sought to rely on his own expert evidence, having previously submitted a valuation report. Rather unsurprisingly the judge ruled that the same person could not perform both roles.
The judge pointed out that the distinction between an advocate and a witness is crucial. He made reference to the Civil Procedure Rules but also to the older authorities on which Part 35 is based. He gave three reasons why an expert cannot be an advocate.
The judge regarded those authorities as binding and 'an absolute bar' to an advocate also acting as an expert witness.
In fact, the report was, in any event, deficient and its contents tainted by an apparent lack of knowledge of the duties owed by an expert to the tribunal. The deficiencies included the failure to provide a statement of truth and a statement indicating that the expert understood and had complied with his duties.
The decision itself is hardly surprising, even the fact that the issue arose at all. The distinction between advocate and expert witness is fundamental. The roles are very different. An expert's role is to assist the Court and give independent, objective evidence. That is not compatible with advocacy.
The lesson for experts is not so much that they should not appear before the Court as an advocate – which they are unlikely to do. But it may be a helpful reminder that, in giving evidence, they should not stray from their independence and give the impression that they are arguing a case.
Read more from Paul Sankey on this topic in 'Expert witness duties: independence and impartiality again', 'Seven classic principles for expert witnesses', and 'Independent expert witnesses'.
 Neil Pickles/Sharon Pickles v Revenue and Customs  UKFTT 195 (TC)
 National Justice Campania Naviera SA v Prudential Assurance Co Ltd (the "Ikarian Reefer")  2 Lloyd's Rep. 68 (Comm Ct), citing Whitehouse v Jordan  1 WLR 246 at 256
 Ibid, citing Pollivitte Ltd v Commercial Union Assurance Company PLC  1 Lloyd's Rep 379 at 38