Ok, thanks for the overnight advice guys.
It started at 0130 last night so I got up and turned the thermostat off. As soon as the water flow stopped the tapping stopped. At this point the existing hot water in the radiator was still causing it to heat slightly, but there was no water flow. So now I'm wondering if it's related to the water flowing
through the radiator rather than the expansion.
I'll talk to ex father-in-law about the other solutions though, I've got to do something because it's driving me mad. I'm cold enough at night as it is without having to turn the damn radiator off
