MC,
re: "the legality of flying a UK Permit Yak over France as 'permits' are only good for the country of registration and do not infer overflight rights of other countries"....
Took a UK registered one across France and Spain a couple of weeks ago but we did have a previously-obtained overflight permission. I wasn't involved in obtaining the thing, but if you want to know how it's done I'll ask the man who did it.
We probably have a year or so's usage left before we hit 500 hours on the current engine.