Worcester 24CDi - Problem with Water inlet?

We've recently been having problems with our system losing pressure. The process of re-filling the system is pretty straight forward and have the filling key. What I've discovered is that this recent activity and use of the water inlet valve (grey knob in manual, but actually plastic nodule with metal key) has worn the plastic pin down - rendering it impossible to turn! Does this need to be replaced? How much will this cost? The problem of the system losing pressure has now been intensified by a piece of engineering stupidity! Please help!