Your radiator has air trapped at the top so the hot water can't circulate efficiently. You'll need to bleed the air from the radiator, which should be quite straightforward;
Turn off the Circulation Pump/Boiler.
The radiator has a Bleed Valve at one of the top corners (it's a small, square ended bolt in the centre of the round blanking plug).
Using a Radiator Bleed Key (cheap from any DIY store or plumbing supplier), turn the Bleed Valve anticlockwise about quarter of a turn until you hear a hissing sound as air escapes. Keep the key on the valve, and when the hissing stops and water starts to dribble out, close the valve tightly.
Use a rag to catch the drips, and have a jar or something handy to catch spurting water if you open valve too far.
Doug, May 2009
