Thank you both very much.

With your help, reading til 1am and a bunch of youtube videos, I finally understand what a floating caliper is and how it works.

I just went back out to the bike, took the caliper off and had another fiddle. With a little bit of force I was able to press the pistons back in with my thumbs, move them out by pumping the lever, and press them all the way home again. Good stuff. I then tried tried sliding the caliper on the slide pins. Nothing. A couple of seconds of light tugging and it suddenly freed off and was sliding smoothly again. A quick bit of reassembly and I could see the slight caliper movement when I used the lever. That wasn't visible before.

A quick spin round the block and braking was restored. Not like a real bike, but as good or better than my XT has ever been.

Back on the drive, I heaved the bike back on the centre stand and was able to spin the front wheel freely with my foot.

The only niggling doubt is whether the disc should still be so hot. I'm literally only doing half a mile at 20mph with regular fistfuls of front brake, so maybe that is normal heat generation, but i certainly can't hold a knuckle on the front disc. I'll try again later with a more normal run and see if the disc air-cools. I'll also take the GS out this weekend and see how hot the front discs gets.

Thanks again guys. Great forum.