What is the purpose of a Fitness to Plead assessment?

Fitness to Plead in the Crown Court (Archbold 4-166a to 4-185)

The purpose of this procedure is to strike a fair balance between the need to protect a defendant, who has, in fact, done nothing wrong but is unfit to plead at his trial, and the need to protect the public from a defendant who has committed an injurious act which would constitute a crime if done with the requisite mens rea (R v Antoine [2001] 1 AC 340).

The procedure is set out in the Criminal Procedure (Insanity) Act 1964 as substantially amended by the Criminal Procedure (Insanity and Unfitness to Plead) Act 1991 and by sections 22, 24, 25, Schedule 2 and the provisions in respect of unfitness to plead and insanity in Schedules 10 and 11 to the Domestic Violence, Crime and Victims Act 2004 for all defendants arraigned after 31 March 2005. Transitional provisions are set out at paragraph 8 of Schedule 12 to the 2004 Act.

The procedure has two stages:

1. Whether the offender is under a disability i.e. whether he is “unfit” to plead (section 4 Criminal Procedure (Insanity) Act 1964) ; and if so

2. Whether he did the act or made the omission charged against him (section 4A Criminal Procedure (Insanity) Act 1964).

http://www.cps.gov.uk/legal/l_to_o/mentally_disordered_offenders/