Several recent cases have addressed this contentious area, particularly concerning experts making statements of opinion reasonably related to facts within their knowledge and comments based on their own experience .
The Supreme Court’s judgment in the case of Kennedy v Cordia (Services) LLP  UKSC 6 given on 10 February 2016 provides a very thorough, and almost philosophical, review of the role of the expert, and provides a useful checklist for lawyers particularly whether opinion evidence concerns factual material. The case confirmed that expert evidence may extend beyond opinion evidence to deal with factual matters.
The judgment sets out detailed guidance regarding the admissibility of expert evidence, the responsibilities of the party’s legal team and the court in relation to such evidence, and the importance of economy in litigation.
The court referred to the 1984 South Australian case (R v Bonython (1984) 38 SASR 4), which set out guidance on the admissibility of expert opinion, which falls into two parts: whether (a) a person without instruction or human experience would be able to form a sound judgment without assistance; and (b) the subject matter forms part of a recognised body of knowledge and whether the witness has acquired by study or experience sufficient knowledge of value in resolving the issues.
The recent Supreme Court judgment set out four considerations which govern the admissibility of expert evidence:
(i) whether the expert evidence will assist the court in its task;
(ii) whether the witness has the necessary knowledge and experience;
(iii) whether the witness is impartial in his or her presentation and assessment of the evidence; and
(iv) whether there is a reliable body of knowledge or experience to underpin the expert’s evidence.
The Court decided that all four considerations apply to both opinion evidence and evidence of fact where the expert witness draws on the knowledge and experience of others in addition to personal observation or experience.
The judgment acknowledges that where experts stray into inadmissible opinion on legal matters the experienced judge will recognise this has occurred and make up his own mind.
Within my own area of expertise, I am often required to point out to the court the relevant factual evidence contained within the large volume of accountancy and financial evidence that is usually presented or to collate this evidence into a form that is helpful to the Court. I may then go on and provide an expert opinion based on that evidence, with the result that distinguishing between evidence of fact  and opinion in some cases may be elusive.
 E.g. Hoyle v Rogers  EWCA Civ 257,  Q.B. 265, CA); Multiplex Constructions (UK) Ltd v Cleveland Bridge UK Ltd  EWHC 2220 (TCC)
 Practice Direction 35.2(4) requires the expert to make clear which facts are within their own knowledge