Thanks for the helpful pointers all .
Easy job. Clamp flexi,Slacken the caliper to carrier bolts, slacken union, then undo the previous bolts, then spin the caliper off the flexi. Then bleed off, a eezibleed is about a tenner on trade usually, if your working on your own.
Wouldn't fuck about with the piston, it'll more than likely leak anyway, the plating usually comes off and fucks the seal.
Going by the pictures, replacement calipers come with the carrier- the caliper to carrier bolts are those funny 5 sided ones that I've seen other people asking about...and mine look shagged, I'm not sure they'd come off even if I had the proper socket, which I don't!
I think what I'll do is just rub the piston down and shove it back in slathered in red rubber grease, just to get me mobile. It should last a 100 mile round trip if I'm careful, I reckon. In the meantime I'll order a couple of caliper refurb kits from Bigg Red or similar, about 32 quid for both calipers, then will do the lot when I change the discs and pads on the other side (I'll grease both sets of sliders this weekend, though).