Thank you for your comments. It may be easiest if I run through some recent incidents which I cannot understand. Firstly, when I go to fill up the car (which is becoming a frequent occurance!) the alarm will go off shortly after I close the doors and begin to fill up. This happens both with the key in the dash, and with the key removed but the car not locked and no-one else in the car. Also I was stuck in traffic recently so I turned the engine off, shortly afterwards the alarm went off again even though the key was still in situ.
After these incidents, when I later lock the car, the LED will not flash, although usually the next day it's back to flashing again. Not sure if relevant but I lock the car with a single press of the fob, two presses don't seem to do anything.