So, it turned out that it was indeed a flaky door latch. I had to get a garage to replace it as I wasn't sure what caused the original problem for definite.
Everything is back to normal from a security point of view.
The next thing that happened, the next day after I had collected the car, was that the "ABS/Traction Failure" light came on and, unlike previous times, wouldn't go off. This was a sensor going down the pan, so back down to a local guy who has a Faultmate or something similar.
As has been mentioned plenty of times it turned out to be the rear left hand wheel sensor. Fortunately, he was able to get one and get it fitted by the next day. To say I was very surprised is an understatement. However, I am pleased to know that this guy can diagnose and fix stuff on a P38 if I can't do it myself.