Under present-day law, if a question is raised in the Crown Court as to whether a defendant (D) is unfit to plead then the question will be decided by the judge taking into account medical evidence. If the decision is that D is unfit to plead then a jury can be asked to determine whether D ‘did the act or made the omission charged against him as the offence” – Criminal Procedure (Insanity) Act 1964 (as amended).
In R v Marcantonio  EWCA Crim 14 the Court of Appeal (Criminal Division) reiterated the law regarding unfitness to plead - see paragraphs 1 to 9. The court stated -
Whether an accused is fit to plead is determined