Received the stuck caliper back from the brake place after they disassembled it and hydraulically pushed the pistons out. It was definitely beyond the tooling that I have available. It turns out that it was mainly gummed up with something resembling petrified brake fluid and a small amount of corrosion after the seal. With a bit of polishing with fine wet and dry paper, removal of some rusty looking stuff from the housing after the seal and reassembly with a smear of Bendix ceramic caliper grease it's as smooth as they come.
Yesterday it was raining cats and dogs horizontally so the shed beckoned. I tried mounting the caliper, however the thread in the diff was corrupted in one mounting point, requiring the disassembly of the rear hub to access and tap out. Luckily I found a 7/16" UNF tap in my collection and cleaned out both threads.
While I was in there I checked the bearing (pristine!) and seal (probably will need replacing) as well as the handbrake system. As you'd expect, the handbrake mechanism was a little sticky, however a clean up with brake cleaner followed by a little ceramic grease, sparingly applied, the handbrake works well.
So everything is now assembled and the brakes bled on that circuit. The brake pedal is nice and firm like it should be. One step closer to being legal to drive to the paint shop to have the front resprayed and a few touch-up items completed.
Some items to clarify: both rear springs will be cleaned and painted this summer and that jack was not used to support the car (a solid jack plus another solid jack stand was used to support the vehicle).
