Ian, your advice was perfect! The pin was indeed stuck 'in' once I got in there - once I'd cleared the dust and rust and taken a little WD40 and a pair of pliers to gently pull the pin outwards, all was well. I can feel it warming as we speak!
Glad someone still knows what to do with the...
I have one cold radiator in my house, and after some research it looks as though the issue might be a stuck pin in the TRV.
However I can't work out how to remove the cover to get at it!
The brand seems to be Myson. It looks pretty retro - no obvious collar that twists, it feels like one...