My advice is firstly to obtain a new wiring harness made especially for the 403. It will have all the correct terminals on each wire termination and each wire will be tagged with connection details. Such harnesses are available from Vintage Wiring Harness in Australia, and are excellent. VINTAGE WIRING HARNESS
http://www.vinwire.com.au
1953-55 BRISTOL 401 & 403 (BR535)Price:$1,690.00 ex. GST $1,859.00 inc. GST
The next big item is getting the correctly fitting rubber grommets for the body holes and getting the stiff loom through them and in the correct location along its length so that the various 'arms' of the loom fall naturally through the correct parts of the body. Once the centrl part of the loom is in place, the rest is a matter of haveing a selection of the correct loom clips to keep it in place. Good luck. A wiring diagram is not particularly helpful or necessary, if you first have the correct loom which is made exactly for the car.