Rear axle cover plate for MM

[Mr Spigot]

Having taken the rear seat out to try and locate the source of an irritating rattle, I discovered that the circular cover plate for the back axle filler was missing some of its retaining parts and flopping around. Does anyone have a replacement or know where I can get one? I have looked in 2 parts books I have but it does not seem to be listed. They are (as far as I know) only fitted on MMs (and possibly Series II). They are shown in the workshop manual and owners manual and I attach a photo of the illustration (it is the round plate, not the filler plug, which is arrowed).