I got behind a woman in an Audi the other day with the same sort of thing happening. The fasteners in the middle of her plastic shield had come out and whenever she got over 40mph, it began flapping up and down, scraping the road then slamming up against the bottom of the car. Kinda looked like Pacman or a fish or something.
When she slowed down, it stopped and went back up to where it belonged. I’m sure her’s has broken off by now too.
Nylon ties.
Okay, that's a bit reassuring. Thanks for your reply.