Righto, tried that 3 times and now I'm confused...
Each time, I screwed the bolt in from the back of the caliper after cleaning up the threads and when I then screwed the bolt in the correct way with the caliper off the bike, it screws in all the way with no problems. As soon as I fit the caliper to the bike and try to screw in the bolt though, it crosses up again and jams after a few turns. I have been extra careful to make sure I've got the bolt in straight so I cannot figure out WTF is causing the thread to cross up.
Think I might admit defeat and take it to a garage in the morning. I don't have a tap & die kit anyway so if it needs re-threading I've got no choice really...
I'm just mightily pee'd off with myself cos I've had the caliper off loads of times before to service it etc and I haven't done anything different this time when trying to refit the ****ing thing!!