B2 Clutch Master Cylinder

Hi, I acquired my 2 litre 90 just about a year ago and after carrying out some peace-of-mind work, have been happily using it several times a week. The clutch pedal has been feeling off for a little while now and I have found that the master cylinder has developed a leak.  Repair kits are available, but having removed it I can see it has been replaced before, as it is has no factory stamps or part numbers. So, I am reluctant to repair it as I have no idea how long it has lasted and anyone who has done this job on their car knows that it's not one that you want to do often! 

Main dealer is not interested, says it's too old. GSF don't stock them and Europarts only have the slave cylinders. Autodoc list a number of options, but it is very difficult to identify the correct one. I have asked the question of them on-line, but no response so far. I am going to replace the slave as well and there seem to be no availability issues with these yet.

Has anyone got a recommendation on parts suppliers please?