Just to update everyone, especially anyone who also comes across this in future since I found nothing on this specific master cylinder.
Then I usually bang them down on a block of timber and they come out pretty easy.
That will get the lower piston set out. There's usually a Pin behind the Upper Reservoir Seal that retains the Upper Piston.
Yep, bashing the flange part on a hard edge caused the first piston to come out. Though again due to documentation we weren't sure why the other one was staying put, There's definitely an upper pin I bet holding the top piston in place as that's not coming out.
Referring back to my original post of course the pin that holds the reservoir in is sticking well in the cast metal body. Awaiting more elbow grease for that one, had enough for today.
Regarding replacement, it may be most important part of the car but so is any safety conscious part, if we're going to go with that then we'd need an entire new car as no classic Mini survives any collision well.
As I mentioned before the front most important circuit on the discs works fine, we are starting to think it was just build up of grime blocking the lowest part of the reservoir and clogging our rear circuit, all of our seals look fine, we had no leaks and the bore looks perfectly clean past the surface grime from being in the engine bay.
Thanks for the advice and useful links, as always very much appreciated.