The use of expert evidence in preparing for proof has come under renewed scrutiny of late, which is perhaps not surprising given the very substantial fees that are often charged by expert witnesses. Skilled witnesses do, of course, have to be certified, either by agreement or by the court, before their fees can form a good charge in any judicial account. In the Court of Session, the issued is governed by RCS 42.13A which provides that, if so certified, charges shall be allowed for any work done or expenses reasonably incurred by that person which were reasonably required for a purpose in connection with the cause or in contemplation of the cause.
Certification will only be granted if (a) the person was a skilled person; and (b) it was reasonable to employ the person.
From a pursuer's perspective, there are usually two opportunities for the defenders to object: (1) there can be an objection to the witness' evidence being led at all on the grounds that the expert does not have the relevant expertise; or (2) there can be an objection at the stage of certification being sought.
There is a body of caselaw which has, over the years, considered the nature and function of an expert witness, most recently re-explored in the case of KENNEDY V CORDIA (SERVICES) LTD  CSIH 76. In summary, the witness must be giving evidence on a subject which the judge would only be able to understand with the input of that witness' expertise. The expert's personal views are irrelevant, and it is not for the expert to usurp the functions of the judge or jury.
'The subject matter under discussion must be necessary for the proper resolution of the dispute, and be such that a judge or jury without instruction or advice in the particular area of knowledge or experience would be unable to reach a sound conclusion without the help of a witness who had such specialised knowledge or experience… The subject-matter in question must be part of a recognised body of science or experience which is suitably acknowledged as being useful and reliable, and properly capable of reaching and justifying the opinions offered, and the witness must demonstrate a sufficiently authoritative understanding of the theory and practice of the subject.'
Lewis, in Manual of the Law of Evidence in Scotland states;
'..there should be in the witness an extensive and accurate state of knowledge and experience of the subject involved, derived from study or practice or both.'
There are, of course, many individuals holding themselves out as experts in all manner of disciplines, and Directories published to assist in identifying an appropriate expert which is, itself, not always an easy task. Nonetheless, it is essential to give careful consideration to the issues you require your expert to address, to ensure that he has extensive and relevant knowledge of those issues, and that the issues in question are not matters which are properly for the judge or jury to decide.
Beware the hasty letter of instruction! You may find your expert is cross-examined on what precisely he was asked to do by you. Ensure he has all the relevant documentation necessary to prepare his report; otherwise, your opponent may well argue that his evidence is flawed because he had not been made aware of X or Y.
The wrong expert may lose you your case - and a non-certified expert will seriously dent the expenses you can recover!