It's possible that Airfix went with the later colours, when the cockpit changed to a mid grey colour on the F3 and F6, which were originally black. However, as far as I know, the seat was still black, originally with a sandy brown parachute "head box"pack and seat back and squab cushions, later changed to a mid olive green colour.
For your period, I'd go with black for cockpit and seat, and the sandy brown parachute pack in the top of the seat.
I'm still in Norfolk at the moment, so don't have access to my photo files or library until after 30th September, so I can't post example pics.