I'm refurbing the front brake.
New pads.
New oring in calliper, piston cleaned etc
New hoses.
Then the new master cylinder kit.
I read up how people have attempted this in the past which helped loads - thanks all.
The one thing in particular that I found really difficult was getting the circlip out of the groove.
I bought some long nosed circlip pliers and filed noses down to help get in there.
This helped but, best of all was getting the piston fully depressed which makes access to the circlip much easier.
Here's my Heath Robinson way of doing it.
Bung in brake lever pivot bolt into master cylinder body.
Mount in vice
Use screwdriver to lever piston all the way to bottom and secure screwdriver handle with can/small acroprop etc.
Now you can get at the circlip.
Hope that helps