Patten J. said,
5. Under CPR Part 35.1 the court has a duty to ensure that expert evidence is restricted to what is reasonably required to resolve the proceedings. To this end the court needs to be satisfied that the expert testimony is properly admissible and will genuinely assist the judge in determining the matters which are in issue and the burden of establishing that rests upon the party who seeks permission to adduce the evidence.
The admissibility of expert evidence is governed by section 3 of the Civil Evidence Act 1972 which makes the opinion of a person “on any relevant matter on which he is qualified to give expert evidence” admissible in evidence.
Section 3(3) of the 1972 Act provides that “relevant matter” for these purposes includes an issue in the proceedings in question. In the context of claims in negligence an issue has often arisen as to whether the expert may legitimately go so far as to express a view as to whether the conduct of the person in question fell below recognised standards of competence or whether such evidence ought to be limited to more generalised guidance on the usual practice and procedures adopted by professionals in a similar situation.
6. On the question of admissibility I am content to adopt the approach set out in the judgement of King CJ in the Australian case of R v Bonython  SASR 45 at page 46…:
“Before admitting the opinion of a witness into evidence as expert testimony, the Judge must consider and decide two questions.
The first is whether the subject matter of the opinion falls within the class of subjects upon which expert testimony is permissible. This first question may be divided into two parts:
(a) whether the subject matter of the opinion is such that a person without instruction or experience in the area of knowledge or human experience would be able to form a sound judgement on the matter without the assistance of witnesses possessing special knowledge or experience in the area and
(b) whether the subject matter of the opinion forms part of a body of knowledge or experience which is sufficiently organised or recognised to be accepted as a reliable body of knowledge or experience, a special acquaintance with which of the witness would render his opinion of assistance to the Court.
The second question is whether the witness has acquired by study or experience sufficient knowledge of the subject to render his opinion of value in resolving the issue before the Court.”
Edited for ease of reading. A fuller extract from Bonython, giving a detailed explanation of the Australian position appears under the expert evidence tag.