Worcester 240 - one cold radiator?

Hi,

I'd be grateful for your suggestions on the following problem - I's got me stumped!

My Worcester 240 combi feeds 5 radiators - 3 upstairs and two downstairs. The boiler pumps the CH water upstairs and the 2 downstairs radiators are then fed from upstairs pipework. Each has it's own supply.

Recently the largest of these 2 downstairs radiators has not been heating. I can feel the hot water in the supply pipe - it's hot at the top near the ceiling and gets cold about halfway down the wall. If I turn off all of the other radiators then this one will get hot. When I turn them back on again the raidator cools and does not re-heat.

I have bled all radiators in the system and don't know what else to try. Could it be the sign of a failing pump perhaps?

Any help, comments or suggestions gratefully received.

Thanks

Andy