glow worm 24 cxi filling loop not working?

Pressure had dropped due to bleeding. Bled again (not used much during the summer) and pressure dropped below 0.7 bar (should be 1.0 bar). Red light came on and boiler cut out. However, turning the filling loop knob has no effect at all. According to the instructions it's as straightforward as turning it gently until pressure rises appropriately - does this mean the valve is broken? How much are new ones likely to cost do you think?