The problem was that over the years, the hole in the box section on top of the plate had worn larger and larger -- to such an extent that the screw could fit through it. This meant the plate would usually fall off when I tried to lift the mechanism, and I was terrified of it hitting the prop, or of losing it through the hatch. Putting it back on was also tricky.
So Heyford Fields have welded on a couple of semi-circles of metal to restore the hole to its original size. They've also gone to the trouble of painting the whole thing.
We've been pretty impressed with Boating Leisure Services at Heyford Fields. They did everything we asked them to do, and nothing was too much trouble. It's not surprising they always seem to be busy.