Sorry to repost, but not convinced my previous post will be seen. How much is a plumber likely to charge to change a TRV that isn't working (radiator blasts heat even on frost setting)? Is it something I can do myself?
You might be able to replace the cartridge, which is the part which operates the valve. I don't think you'll be able to buy just the cartridge but if you can buy an identical valve to yours you can take the cartridge from it and replace yours. This is a simple replacement - no draining needed, just unscrew yours and replace it with the new one. Just like putting a new cap on a bottle - 5 minute job.