Hi, I have a problem with my upstairs radiators. All of the downstairs ones are lovely and hot, but the upstairs ones are cold. Except for the heated towel rail in the bathroom which is hot halfway up. I have tried to bleed the radiators, but water comes out of all but one of them. Air comes out of the one in the bathroom and the top of it gets hot, but then a few hours later, the top gets cold again. Some of the radiators that are cold have one hot pipe going into them, but the others have two cold pipes. Is there some knack to bleeding radiators that I am missing, or is there some other problem. Pleese help. It's absolutely freezing!