SOLVED:
Finally fixed it! Went back to basics and simply disconnected the battery for 30 seconds, then once reconnected locked it - the alarm went off once and never since. I have also re-programmed two keys with it (as one was programmed before disconnecting) and no problems.
Not sure what was causing the problem - perhaps some memory within the ECU or alarm was scrambled and disconnecting the battery cleared it?
Thanks for everyone’s help - hopefully this solution will help if anyone else encounters the same weird problem!
Andy