CPIA 1964 s6
Evidence by prosecution of insanity or diminished responsibility.
6. Where on a trial for murder the accused contends—
- (a) that at the time of the alleged offence he was insane so as not to be responsible according to law for his actions; or
- (b) that at that time he was suffering from such abnormality of mind as is specified in subsection (1) of section 2 of the Homicide Act 1957 (diminished responsibility),
the court shall allow the prosecution to adduce or elicit evidence tending to prove the other of those contentions, and may give directions as to the stage of the proceedings at which the prosecution may adduce such evidence.