Just though I should post this experience, incase it helps anyone else out.
Once the wheel is off the only thing holding the disc onto the hub is two small fastening pins. Unfortunately, they appear to be made out of a rather soft alloy, and they just shear off.
I'm not sure whether this is a general thing, but I've had to take four off altogether, and all four sheared off with very little force.
My discs were in very bad condition, so I think it's possible that they were getting very hot, and this might have contributed.
On the first side I worked on, I attempted to drill out the remains of the bolt, but it seemed to have fused with the hub completely. There was no way it was coming out, I ended up having to drill it to a size where the new bolt would just fit, but very tightly, then use the 'irish screwdriver' technique. ie. smack it with a hammer to fasten it over the disc and spacer.
On the other side having already wrestled for a whole day with a crap drill on the first side, I just left the remains of the bolt in, and rested the disc on them while I changed the pad.
(If anyone else does this, and has the same problem a good tip is, to keep the disc in place while you're messing with the calipers and pads, put the wheel nuts back on, but with a piece of plastic pipe or something around them, so it holds the disc tight.)
It's not so much a problem, as everything is now held together by the wheel nuts, but if anyone other than me is going to need to take the wheels off, I must remember to warn them that there'll be nothing holding the discs on. eek!
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To err is human, to moderate ursine.