Yesterday I drained my heating system so I could cut the pipes on one radiator to raise it. After refitting, I filled and bled all the radiators and they were all working. When I got up during the night to let my dog out, two upstairs radiators were still hot and hotter still when I got up this morning.
We have no hot water tank, its a direct feed system that works only when turned on, so no water is stored in an immersion heater. The heating system was not on. I've tried bleeding the two radiators again just in case, but no air came out just water.