Tony,
you may find some info in the Public Record office. the following being the files I'm aware of (although I've not got round to looking at them yet!):
AVIA 15/317
(AIRCRAFT: Design and Development: Fighters and Fighter-Bombers (Code 6/3): Proposals for development of Westland Whirlwind fighter 1940)
AIR 2/2821
(AIRCRAFT: Fighter (Code B, 5/6): Whirlwind single-engined single seater day and night fighter: specification F37/35
1935-1944 )
AIR 16/326
(Westland P9 Whirlwind aircraft )
1939 Sept.-1942 Sept.
AVIA 46/122
(Westland Whirlwind )
AVIA 15/2360
(AIRCRAFT: Design and Development: Fighters and Fighter-Bombers (Code 6/3): Consideration of design for production of Westland fighter to specification F37/35
1938-1939 )
The PRO catalogue doesn't tell you what's in the files, so it may have useful things like drawings, or a series of letters that are of no practical benefit but which might be of interest.
Also, there is a book (that you've probably either got or seen, but in case not) Victor Bingham, Whirlwind: The Westland Whirlwhind Fighter (Airlife, 1987) ISBN: 1 85310 004 8. This has appendices giving things like wing area, dihedral, etc, etc.
Hope that this may be of some help; good luck!