how do I fix my Danfoss thermostatic cylinder valve?

The Danfoss thermostatic cylinder valve has stopped letting water into the hot water tank.

I have removed the head and the electric switch underneath that stops the pump when the water is at the right temperature works fine.

Can i take the electirct unit off as well without draining the system?

What will I find underneath? - can I mend it at all?

I can't find any useful information!

The heating system is around 20 years old unfortunately.

Any help much appreciated.

Richard
January 2011