I might be too late but for future reference I have just fixed my Myson TRV. I opened the valve fully, unscrewed the knurled ring under the plastic body to release it from the metal valve body. Gave the brass pin a short squirt of WD40 and left it for half an hour. Returned and gave the pin several gentle taps. "Hey Presto", it worked. Best of luck.
Mac
February 2011