Simon,
Regarding the brakes Zeus used to supply upgraded front callipers to Bristol for the earlier cars, these as I remember from talking to them years ago were used in conjunction with the standard rear set up. I think the current brake upgrade that Bristol Cars do utilises AP Racing callipers and may also involve replacing those on the rear, choice and quality of brake pads can make a difference and off course the condition of the brake system itself.
The previous owner of my 603 Edward Hathaway had the brake system altered, the twin servos and separate master cylinder were replaced by a single combined servo and master cylinder mounted on the bulkhead, this was a Ford unit I think off a transit and carries the numbers NR 94
VB 2B 195 EC and under that R5FPA. The brakes are much sharper and positive in operation than the standard set up on a 411 though I always found those adequate for the performance but with a different feel to modern brakes, If I ever get mine back on the road we might copy the modifications on the 603 especially if the twin servos need work.
Geoff.