Page "M'Naghten rules" Paragraph 35

Section 1 of the Criminal Procedure ( Insanity and Unfitness to Plead ) Act 1991 provides that a jury shall not return a special verdict that " the accused is not guilty by reason of insanity " except on the written or oral evidence of two or more registered medical practitioners of whom at least one has special experience in the field of mental disorder.

This may require the jury to decide between conflicting medical evidence which they are not necessarily equipped to do, but the law goes further and allows them to disagree with the experts if there are facts or surrounding circumstances which, in the opinion of the court, justify the jury in coming to that conclusion.