Got the top off with a good wiggle after removing Allen screw and sure enough pin was stuck down. With a little bit of judiciuous tapping up it came and rad is now warm and toastie.
Thanks for all advice given.
I want to check whether the pin is stuck on old Danfoss thermostatic valve ( no model number on it just Danfoss Thermostatic) . There is a lug on the black knurled ring at the bottom of the valve housing with a very small Allen screw which I have loosened off and tried unscrewing top but nothing...