a thermostatic radiator valve sits on the flow pipe end of the rad, it controls the amount of heat emitted by the rad when you turn the head to the various settings, o-5 usually.
this head can be unscrewed and allow access to a narrow pin.you can wiggle this or take it out to clean it from the build up of crap, like when you open your mouse and there is crap on the bars the mouse ball rubs against.
there is a tiny loss of water but the clean up helps cos the pin moves up and down freely allowing the valve to open to let hot water into the rad
if this does not work then check if the rad is hot at the top and cool at the bottom.if so hot water is getting into the rad but not circulating by leaving through the return pipe at the end where the small valve (the lockshield) is because it is sludged up with crap that needs to be flushed out.
i never used to take off rads to wallpaper or paint behind and this was okay untill they sludged up and i was forced to take them off.after this point i took the rads off to decorate,making it easier and a neater job and also carried out some preventative maintenance.just topped up the inhibitor and bobs your uncle.
i think part of the problem is that we never touch things untill they go wrong, instead of say cleaning things every now and then. this seems hard for you at the moment but you are learning as you go along
i,ll check back at half nine. popping out good luck