I have some TRVs which dont work at all when i turn them (the flow to the rads remains unrestricted = very hot). Is there anything i try to rectify this? Or do i need new TRVs?

most home owners turn down their TRV's in the Summer months. when Winter arrives, they turn up the TRV's only to find that some may not work.

Undo the head of the TRV in question. once off, you will find a silver pin. this is likely to be stuck in the closed position. lubricate it with WD40 (minimal spray), then use pair of pliers to pull pin upwards. Use flat screwdriver (or similar) to push pin up/down.
viola, your TRV is up and running. NO, well time to replace it then. Sorry.

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Men will always be kids. Even after they have grown up, they keep trying to go back where they originally came from!