Jammed thermostatic radiator valve - how to free?

[zebedee]

I have some fairly old Honeywell thermostatic radiator valves. Some of them won't free up unless I hold a rope burner (basically a 1000 degree c cigarette lighter) onto the pipe to get the stone cold water under them (even though other rads are already by thennice and hot) and they they crack open with a loud crack. One in particular however oddly always has a very hot supply pipe but won't open anyway, or if it is it is only a tiny amount as the area next to the valve is warm but not the rest of the rad. That could just be conducting along the metal of the supply pipe, valve and radiator though. I have tried wiggling the control from off to full many many times in quick succession, trying to pull and push on it and all to no avail. Is there anything else I can do other than replace it (probably with a manual one to be honest!)

cheers