In reply to Franco Cookson: Presumably there is some flow through it else the pipe wouldn't be getting hot (unless they are relatively short T's of the main flow and return in which case they could just be conducting heat from there. Some flow but no heat in the rad says two things to me
1)It may need flushing as it is full of sludge - take it of and flush with a hose, if the system is a closed one you'll need to top it up after refitting.
2)System may need balancing. Turn off all the other radiators and see if it gets warm then. The water in the system will always take the path of least resistance and if the pump is on the small side for the system you may need to restrict the flow in some of the other rads in order to force more through this one.
Note I'm not a plumber although I did install our CH system several (problem free) years ago. Advice taken at your own risk and all that . . .