Hi, can you advise if you use your boiler to heat up your hot water?
Is it just the one radiator or is there a number, and are they in different rooms?
Unfortunately this is abit of a strange one, as the only heat source for the radiator is the boiler, unless you have a stove \ aga which has a back boiler.
If the radiator continues to get warm every morning I would be included to switch the boiler off in the evening, then check if the radiator is cold in the morning, that quickly proves if its the boiler or not.
Just to clarify my last point, when I mean switch the boiler off I mean switch the power off on the boiler completely not just set the timer to not produce heat.