radiactor cold at top?

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