This happened to my friend last week and i was there when the man from British Gas came round. he said that if bleeding the rads didn't work it meant they were full of sludge (his words not mine) and they would need to be flushed out. This is quite expensive, even if you have cover with british gas (as it's not covered in the policy) so bleed them and leave it a few days and see if that helps.

You have described the classic symptoms of an Air-Locked Radiator. These need to be bled, to release the air, trapped inside.

The bleed valve, is typically located at the rear of single panel radiators, or on the inter-panel connection on double panel radiators. You will need a bleed key to do this, if you haven't got one, they can be purchased for about £1, from any hardware store. They can sometimes even be purchased from some newsagents or corner shops.

This will, however, cause your system pressure to drop. So you will need to keep an eye on that, topping up as required