Hi Bazza,
BSA used the same filler cap at around the same time. Theirs have the piled arms logo stamped on , but it easily polishes out since the cap is made of solid brass. I have also seen pressed steel ones at autojumbles; they fit fine and they are usually very cheap, but they don't look right (I might even have one if your really stuck). I think these are oil filler caps from cars, but I have no idea what sort.
Glad you've solved the engine mystery.
Good luck,
Stuart.