I had this after I tighten the knob too much. I fixed it by turning the water off and reducing the pressure. The value is only held in by some metal clips remember where they go, take them out and a cloth is handy not to be soaked. you can the take the valve apart. I had the small plastic stopper stuck. Reassemble put into boiler and reclip. turn water on and it should work. It also stopped once because the mains inlet pressure was too high. try turning the cold water tap before filling your system.
Neil
November 2007