An update on this - I finally got the chance to swop out the master cylinder and servo assembly at the weekend. Early days, but it is so far = so good. If it does prove out I'll be pretty happy I don't have to go down the route of the ABS - apparently you need to program it on the car, which I don't have the software for, therefore would have to transport it to the nearest Xpart garage (~35miles) which is difficult when your car can't be trusted to go more than a couple of miles. Apologies if this is tedious - I just want to close the loop on this thread.
Related subject - I don't see any "How To's" on the ABS in general - best way to purge it, bleed it, program it ............