This, plus the fact I was trying to sort out problems with the brakes made me call it a day early.
I started by trying to remove the snapped off bleed nipple in one of the calipers. I drilled a wider hole in the centre and tapped in a small Torx bit. Massive amounts of heat and penetrating fluid had been given lots of time to do their stuff, but the Torx bit still managed to snap off in the hole. Then I drilled it as wide as I could and tapped in a larger bit but this was not working either. In the end I gave up and pronounced the caliper scrap.
After a dig around in the back of the cave I found another caliper and started the disassembly process again. The 2 bleed nipples came out easily on this one, and then I split the caliper and removed the pistons. Again, much heat was required before they surrendered.
I then cleaned the caliper halves in the parts washer and dried them off. Tomorrow they will get a ride in the ultrasonic bath.
And that was the end of the work today.