The requirement for expert evidence is almost taken for granting in construction and engineering cases, where it is assumed that the technical nature of the dispute is sufficiently complex that the court will require help understanding it. However, expert evidence is not a mandatory right as explained in the case of British Airways Plc v Spencer and 11 others (present trustees of the British Airways Pension Scheme)  EWHC 2477 (Ch).
CPR 35.1 ("Duty to restrict expert evidence") provides:
"Expert evidence shall be restricted to that which is reasonably required to resolve the proceedings."
The White Book notes (at paragraph 35.1.1) that the underlying objective is to reduce the inappropriate use of expert evidence.
The courts have adopted an increasingly stricter line on this, particularly following the Jackson reforms. In Andrew Mitchell MP v News Group Newspapers Limited  EWHC 3590 (QB), Warby J considered whether it would be appropriate to admit expert evidence:
"CPR 35.1 imposes a duty on the court to restrict expert evidence to that which is reasonably required. I do not, however, read this as imposing a test of absolute necessity. A judgment has to be made in the individual case, and it has to be made before the evidence is heard and evaluated. My conclusion was that evidence which it is credibly said could conclusively determine the single most important issue in the case meets the criterion in the rule."
In British Airways Plc v Spencer and 11 others (present trustees of the British Airways Pension Scheme)  EWHC 2477 (Ch), the deputy master refused BA leave to call expert evidence. He held that "various points raised in the pleadings" on which BA suggested expert evidence would assist were, in fact, "eminently capable of being determined by the judge at trial as issues of fact and law without the assistance of expert evidence...". BA appealed. The appeal (which was decided by Warren J) succeeded in part. It set out the following principles
- Whether, looking at each issue, expert evidence is necessary to resolve it. If it is necessary (not just helpful), it must be admitted.
- If the evidence is not necessary, whether it would assist the court in resolving the issue. If it would be of assistance, but not necessary, the court could determine the issue without it.
- Whether (in circumstances where the evidence would be of assistance but not necessary), in the context of the proceedings as a whole, expert evidence on that issue is reasonably required to resolve the proceedings (taking account of factors such as the value of the claim, the likely impact of the judgment, where the costs will fall, and the possible impact on the conduct of the trial).
Although the decision was particularly fact specific, the court identified the following points in favour of allowing expert evidence:
The "very large" financial implications of the decision for BA meant that it should be entitled to advance its best case.
- It would be undesirable to "tie the hands of" the trial judge.
- If the trial judge concluded that expert evidence would not assist, then he could decline to receive that evidence.
- The fact that BA would bear the costs of the expert evidence whatever happened, and that the trustees would bear no financial risk.