CAP 804 section 1 part E.10
'Heavy' Crew
When an aircraft crew consists of more than the required number of pilots (i.e. a 'heavy' crew) the rules for logging of flight time are as per Part-FCL.050, but for clarification the following should apply:
10.1 Pilot in Command
The designated commander of the aeroplane may log as pilot in command all the flight time. This includes rest taken on board.
10.2 Co-Pilot
The designated co-pilot of the aeroplane may log as co-pilot all the time he acts as co-pilot whilst sitting in a pilot's seat.
He may log as PICUS all the time he occupies a pilot's seat and acts as pilot-in-command under the supervision of the pilot in command or a cruise relief pilot substituting for the pilot in command.
He may also log as pilot in command all the time he is acting as pilot in command and substituting for the designated commander of the aircraft when he is taking rest.
He may not log as flight time any period s during which he does not occupy a pilot's seat.