[derekgas]

Sorry I have only just seen this, maybe it is sorted out by now? Had a few personal issues which some on here know about, anyway, if you have not sorted it out here is my opinion.
The return pipe from the radiator to the heating main pipework is blocked, you may find that the pipes are run up the wall rather than fed from underneath, putting chemicals in when it has got so bad isnt going to work, either have the radiator changed and the pipework to it, power flush the system or both. It is your landlords responsibility to do whichever, holding back rent will get you in trouble, go the proper route as suggested with the council.