Tax is complex and the financial consequences of a mistake can be catastrophic. Deciding where the legal liability for the consequences of such a mistake lies itself calls for high levels of professional knowledge and judgement, and those who specialise in litigation cannot be experts in revenue law and practice.
The court, in reaching its decisions in such cases, must form a judgement on many matters requiring professional experience in the practice of taxation. In cases concerning negligent tax advice, the litigator and the court require the aid of an expert.
The courts have heard expert evidence at least since the sixteenth century (see Buckley v Rice Thomas (1554) 1 Plowd 118).
An expert witness falls into a special category governed by Part 35 of the Civil Procedure Rules (CPR) and the Practice Direction ‘Experts and Assessors’. Guidance on their application is given by the Civil Justice Protocol for the Instruction of Experts to give Evidence in Civil Claims.
An expert witness will owe a contractual duty to the party engaging his services under normal contractual principles, but his overriding duty is to help the court in matters within his expertise.
Under CPR, Part 35.1, ‘expert evidence should be restricted to that which is reasonably required to resolve the proceedings’. Without the court’s permission an expert cannot be called to give evidence and an expert report cannot be put in evidence (see CPR 35.4 (1)). Even if the evidence is admitted, R v Rivett (1950) 34 Cr App Rep 87 determined that the weight attached to it is a matter for the court.
However, in Samson v Metcalfe Hambleton & Co  26 EG 154, it was held that a court should be ‘slow to find a professionally qualified man guilty of a breach of his duty of skill and care towards a client… without evidence from those who are in the same profession as to the standards expected on the facts of the case and the failure of the professional qualified man to measure up to that standard’.