OK so here's an update,
the alarm is still going off, even this morning at work - its a nice sunny day so the cold must not be the issue.
next up was the battery. i have had no issues with starting etc so couldn't see this being the issue, but in the face of science and angry neighbours i popped to Halfords. the tested it for free would you believe it! (*considering they sting you £3 to change your wipers lol). After many car compliments, the results were in - 409/500. very good battery!
Right next plan was the Air conditioning. I haven't ever cleaned it out, so this idea was a nice one even if it doesn't fix the issue. So I bought a bomb! This was set off 20 minutes ago, and went for 10 minutes. By god the car smells fresh! I am currently sitting in my office chewing on half a french stick filled with salmon, and have given the warehouse boys stick instructions to report back to me if it goes off again.
Alio, I have only just seen your message, and if it goes off again, your solution will be next on the list, so thankyou