How do I fix Sar radiator valve?

OUr bedroom radiator is stone cold. It has a small plastic cap on the valve which you turn to increase or lower the heat. There is a pin underneath this which goes down to close the radiator off and should pop up to let the heat back in. The pin will come up only if lifted but there is still no heat coming into the radiator. I've tried tapping it in case it is stuck. Does the whole valve need to be replaced? Can it be done without draining down the central heating system?

Lucy
January 2013