Well, bizzarely, it works now. Rick, the gearbox guru called me (thanks Ade) and talked me through what was likely to be wrong, although most of the conversation involved me looking at a gearbox blankly while Rick described components I've never heard of or seen before. Everything he said was already as it should be, so I put the top back on, and it works. I've yet to go and take it up the road (because I'm too much of a pussy, and it'll probably break) but a little bit of trying it5 up the drive and it seems to be ok. I still have no idea what I did wrong, or to be honest, whether I've fixed it properly, but there you go...