Thanks 147cjl / Artermis, I think I'll try this and open the doors first, although it doesn't answer the 'why only when it's cold' question...
Satanism - I have 3 buttons - main doors unlock, main doors lock, and boot release.
Thanks Ellaura for confirming the sensors / locks relationship...
I always unlock doors when I'm going to open them, and not before, so it's definitely not that it's re-locking before I get to it...
The main alarm does not (and should not) normally go off if I just open the boot. So logically, this would suggest that if the alarm goes off when I open the boot without opening the doors, the alarm still thinks
the car is all locked.
Conclusion: from all your input, it's not disarming the cabin sensor, even though it's releasing the boot lock - does anyone know where the cabin sensor is?? (ie why it would be bothered by the cold)
Thanks guys'n'gals - I think we're getting somewhere