Switched on the FBH (Fuel burning heater.) by remote, and loaded up the car.
Heater ran for around 7 minutes and we were ready to go, so I tried to start. Engine turned over and sort of fired but would not run. I did notice that the glowplug light was not on. Bear in mind it had been round -12C overnight. Eventually, after loads of cranking, it ran. It had been hesitant the morning before, again having run the FBH for only a few minutes.
Now I'd not seen any problems with the glowlpugs before, so I wondered what might be the cause. My theory is that the water was warm enough for the control to decide glowplugs weren't needed. But in fact the heat hadn't got through to the cylinders.
Tried it this morning without running the FBH, and it was -9 overnight so very similar. Glowplug lamp came on, instant start.
So the moral of the story is, ensure you start the FBH with enough time for it to get PROPERLY warm, or don't run it at all. It may fool the system into not running the glowplugs.