Thanks Matt. When I am next with the pen I will check the barrel.A quick search of Pilot Elite shows a couple of filler types, including a bellows filler that looks like a button filler with a metal shroud. Obvious question: have you looked up inside the barrel? Yes it appears parts are missing.
Out of interest if it is the bellows filler and the missing shroud is not in the barrel, do you think this could be fixed at sensible cost?