Yep, the master cylinder seems to be top of the list of possible causes. It is at a garage at present which clams to be able to get parts and know the vehicle. I warned them about needing to remove the dash and they said they knew and were relaxed. I see you are Bristol based, this garage is in Hengrove. I'll let you know how I get on.
I did replace the fluid and bleed the brakes. Good for 1000mls then the problem returned.