Having taken off the blue tap, it does appear to be broken. I can now fill the system and it is working but obviously it'll need replacing with a brass one.
Thanks for all the help.
There is no water entering the system so not going anywhere. When I use the pressurising valve on the boiler, there is not the usual sound of water. Only replaced an upstairs radiator and the end valves.
I've just fitted a new radiator and now the boiler, a Glow-worm 38cxi, won't repressurise. The tap I normally use to maintain the correct pressure is having no effect.