Thanks for your reply - I have tried that and tried opening fully.
What I can't understand is that the entry point to the radiator after the valve is very hot!
Have a double radiator in lounge bay window that takes a long time to get hot and only gets hot at the top. The pipe going to the radiator valve is very hot and so is the entry point into the radiator - all other rads are ok and get hot very quickly. - any advice would be appreciated.