G'day all. New to this site (not a plumber) but with a familiar problem, albeit with a twist. Wickes toilet in a box with Siamp internals fitted 5 years ago and has been fine until the last week ... slow to fill, so replaced split diaphragm (well documented on this site), which cured spraying from around connection but now water spurts out of top of fill valve through central circular hole in cap! The washer beneath the cap looks OK but has nothing in its own central hole. Found a plastic plug in bottom of tank but when fitted in hole (and I don't know if it should go there) it just gets blown out by water pressure. Float works fine and shuts water off correctly. Have put old washer back in for the time being as that scenario (slow fill) was preferable to water dripping from lid! I cannot work out how this contraption functions and just wondered if there are any ideas out there before I resort to replacing the whole valve assembly.