I have Myson TRVs also and am having the same problem with a TRV on a bedroom radiator.
The plastic sensor in the valve is broken and cannot be repaired.
You have to turn the TRV to 0 and unscrew the top of the valve and replace it with one exactly the same. This saves you having to change the whole valve, which entails draining down the system.
I got the answer to this question on Screwfix under the plumbing section and DIYnot.com.
These are excellent sites for getting this sort of information
Alex Patterson, January 2007