If loosening the valve is your aim and you have to buy something specific then don't bother with standard WD40, it's ok but there are many better, PlusGas for example, WD40 do make a specific penetrant but I've never tried it. Whichever you use leave for 24 hours and then if you can, tap the screw to encourage the threads to come free.
Thanks Guys. Since I posted, I have been onto youtube and there is a fair amount of help on there, although my radiators are now 35 years old, and aren't anything like as modern look as the one of Youtube. xxx k90v,nglj7i]8p\ m.;lkio
Do you mean the air bleed pin Mickey. Bin the aluminium key if that’s what you’ve got,they’re too soft. Get a good brass one. Failing that and this is the very last remedy. get yourself a rad key a tool that tightens in the whole plug. Turn off both valves and loosen the plug you can get at. You should be able to bleed before the plug comes all the way out.
What you could do is to turn off the two valves and drain a little water off. Then apply heat. A blowlamp should do it. Once it's heated up, the joint should give and undo. Worth a try.