I've searched around a bit but most queries regarding broken TRV's are in reverse where the radiator is cold due to the pin being stuck. Does it also happen in reverse? We have 3 or 4 radiators fitted with TRV's that remain permanently hot regardless of where the TRV is set too. The remaining radiators fitted with TRV's all work fine and can shut off accordingly.
I guess removing all the heads to see if the pin is stuck permanently on is first port of call but is it likely that 3 or 4 would fail with exactly the same issue at the same time or is the whole unit likely to need replacing as valve (or whatever restricts the flow) is broken?
(At an outside guess could the TRV's have been installed on wrong side of radiator, but surely that would be exceptionally unlikely? They were installed some tme in last 10 years and I think it's likely to have been done by plumber as whole system was replaced during an extension build so likely to have been a 'proper' plumber rather than DIY)