Yep - I have finally achieved a higher level of existence - "piston broke"
Seriously, I had a SA63 15/16th MC re-built and bored out to 1 inch in the eastern States (thru a mate) ages ago and finally got around to fitting it to my RA23.
I went to bleed the brakes last night and the rod/pin in the booster has pushed the piston in but it will not return to its static position.