And calum I apologize for stepping on your wanted thread but at least a lot of good info has been shared here and I hope you found a solution to your problem.
No worries
I now have three front master cylinders, two of which are in really good condition, so I'm all set. I also just got a NOS rear master cylinder for 80€ which is so good I'm not even sure I'll put it on the bike.
@lash: If you mean the translucent white reservoir then I put them in a zip-lock bag with hydrogen peroxide based hair products (I use a 12% solution) and leave them in the sun for a few days, turning them occasionally so the bleach gets spread around.