Well after losing my filler cap the original has arrived. To prevent this happening again this is what I came up with.......I know this is theft proof but hopefully it will stop a lose cap disappearing into the countryside.
I drilled a small hole through the centre collar. Then I fed a cable tie through - cut it and crimped an electrical eyelet connector. I secured that with the lower collar screw.
This may save someone from losing theirs or if there's any suggestions to improve the way I've done I'm all ears