Thanks for the suggestion, I had previously tried this before with no luck but I've just been out to try again. I got the confirmatory beep but again after 3 minutes and 33 seconds the alarm went off. I did make an interesting observation though. In my haste to turn the alarm off to avoid being the most annoying neighbour in our road; I had missed the hazard lights weren't flashing. So the siren goes off but the hazard warning lights don't.
To do some more experimentation I sat in the car, set the alarm (normally so the movement sensor should be on), waited 40 seconds (I think it takes this long to arm), I then waived my arms around and the alarm didn't go off. I then opened the door and the siren sounded and hazards did flash as expected. I then entered the ignition code 123 11 and got 2 flashes on the tell tale, which as expected according to this Lotus Service Notes Section MR indicates "Door, bonnet or boot lid" which it was.
When I enter the code 123 11 after the 3 minute and 33 seconds siren but no hazard lights I get no flashes to tell me what caused the alarm.
Faulty siren unit or battery within?