 If a defendant at the time of trial claims he is insane , this hinges on whether or not he is able to understand the charge , the difference between " guilty " and " not guilty " and is able to instruct his lawyers . If he is unable to do these things , he can be found " unfit to plead " under Section 4 of the Criminal Procedure ( Insanity ) Act 1964 . In that situation , the judge has wide discretion as to what to do with the defendant , except in cases of murder , where he must be detained in hospital . 
