Well thanks Guys for all the replies.... but I was a bit lost. Anyway, I found a way to fit it back on on, but then turning the key didn't pop the lock (at least I had the sense to try this before actually shutting the boot!!!
When I took it back off to see why it wasn't working I realised that part of the lock which should contact the cam was bent. So I tried to bend it back and..........
Yes you've guessed correctly - it broke!
Found one on ebay, but if anyone has one of these which I can buy on Sunday at swap meet, please let me know.