After waking the neighbours in the early hours with the alarm going off, it was fairly obvious the battery was flat. I had noticed the fan running when I'd parked in the garage, so I pulled the fan relay, put the battery on charge and went back to bed. Time to replace the thermoswitch in the radiator!
944 606 481 00 temperature switch
N 043 808 1 sealing ring, A 22 X 29
New thermoswitch and seal:

Thermoswitch location, under the top hose, nearside of the radiator:

I used a 29mm deep socket to get the old one out:

New one in place:
