Fix a dodgy radiator valve with this simple ‘how to’ guide.
First, remove ONE of the chrome radiator valve connectors (tails) at the bottom of the radiator. You will need a 12MM Allen key to do this, unless it is one of those with an unusual internal shape. You can obtain the tool for these from most good DIY shops for a few pounds. They are usually quite tight, so you will need some leverage. They unscrew anticlockwise, facing the radiator end.
If you are replacing the radiator with a new one, then remove BOTH these, as new radiators do NOT come with these (new valves SHOULD come with radiator tails though). Attach one of the connectors to the leaking valve, using some PTFE tape around the thread first. Then, find one of those wine bottle keepers (the cheaper plastic sort are ideal, as they are thin enough to go into the inside of the open connector tail, when unexpanded).
Push carefully in, then use the lever to expand. Of course, if you have the correct size cork, that will do just as well. This means you do not have to worry about putting the radiator back for a while, and you don’t have to keep emptying a ice-cream container (drip-catcher). If you are planning in keeping the rad off for a while, then tape the cork or wine bottle keeper in place. I have had one of these in place for 10 days while waiting for a replacement radiator.