I couldn't have put that better myself Nick
You have a P.M.
Take those soppy foam rubber gadget things out of the boot side flap storage areas so, that you can see the inside of the rear quarter panel and the inside back of the wheel arch seams. If there's been a lot of heavy breathing going on inside the car on dark cold wet evenings during the life of the car, surface rust will be in there somewhere, especially at the bottom of the rear lower areas behind where the bumper is fitted... this is also where the sunroof rear drains exit, and the rubber bung around the pipe is usually sat in a puddle of rust. Take it easy getting them out, as they can tear (bonded in), so small prising a bit at a time may save them intact.
The other cheaper solution, that could gain you another couple of years is to clean the outside back to a reasonable finish, but most importantly to give the inside some treatment too, and it will prolong the time it takes for the rust spots to start appearing again. I've used Dinnitrol sprayed on a bed of penetrating oil, and a second coat some time later, inside those side boot areas. One screw out at a time on the rear door catch will allow reasonble access to the inside front of the wheel arch. Regular cleaning and a smear of sillicone grease inside the wheel arch and the back door rubber seal will keep the timworm at bay for longer than you might think.