My uneducated thoughts are that there’s maybe a ribbon wire unit internally maybe that moves/rolls along while zooming to different points and when moved to the 200mm end, it’s revealing a broken wire in the ribbon or at its connection point. Maybe.
It needs to go to Canon. They usually have fixed repair costs in 2 or 3 categories, I'd hope this is the low end repair. Most of the cost comes from the labor to tear down, reassemble, and test or calibrate if that is thrown off by the parts replaced. I'd send it in sooner, its always possible that it could develop into a more serious repair.
Thankyou both, for your time and thoughts.
Canon have said it should be the ribbon or at worst the diaphragm. Great to hear ‘send it in sooner’ I definitely will now, and post the result. I would like to part trade-in for the RF version, so it needs to be done. Fingers crossed.
Its been amazing though, I must admit.