Dead battery but alarm set
This is not so much a question, just recorded experience that might help someone.
Yesterday I noticed that that no electrics were working - the battery was stone dead. I must assume the cold weather caused it to freeze in the handful of days I didn't drive.
But the alarm went off when I tried to put the key in the ignition, or tried to undo the negative pole of the battery. As I understand it, the alarm has a back up battery - and was still able to check power changes that indicated movement of the main battery.
What to do?
What I did was to change the battery as quickly as possible so as not to keep triggering the alarm. The trick is to take of the strap first - by far the trickiest part. I was lucky in that there were no screws on the positive pole cap going into the battery, so the operation was quite swift. I think the alarm gave up before I had finished, but there was no problem in the end.
Whether my alarm is working properly now, I'm not sure - but at least I can drive the car to the dealer to check that.