Hi Peter & Peter!
I've just finished the refurb of the rear callipers, you can get an overhaul kit from Big Redd brakes that includes the pistons and all seals including the rubber boot on the end. Leave the callipers attached to the brake hydraulics and remove from the carrier. Put a block of wood in place of the pads both sides and use the brake pedal to move the pistons outa bit, or a lot. Remove from the car, pull the boot off the back, remove the circlip and push/brift/hammer the handbrake lever assembly out. Catch the wedge and spring and unscrew the actuating rod from the back of the calliper. You can then push the old piston out with a drift/hammer. Clean and reassemble. Which is a bastard of a job. There must be a special tool for this.. I used brute force to get the handbrake assembly back in, the piston is pressed down to engage threads and twisted.
Bleed nipples are also at risk of shearing. I soaked mine in release fluid after scraping the crud away for a week and all six came out cleanly. Almost unheard of.
Alternatively, you can ship the whole lot off to Big Redd brakes and they'll do the job for you.
Guy