Clive,
Accessing any UK aviation regulation easily these days is a challenge in itself!
That aside (and assuming UK law applies to your operation), you'll find it here:
UK CAA website. (The AMC & Guidance Material is accessible too.).
The authoritative source is from The National Archives -
'legislation.gov.uk'.
Depending on whether the aircraft is 'Complex' or Other Than Complex', search in the IDE bit (Instruments, Data & Equipment) of NCC or NCO respectively.
Enjoy...