Hi, just following up on this. Yes, the cause of my issue was the plastic bit that sits where the HUD would be. It got worse and worse and eventually it was vibrating and rattling all the time. Even a small tap on it when the car was stopped would result in a loose plasticky sound. They fixed it during my car service yesterday by adding felt. That's rock solid now and no rattles anymore thankfully.
If only I could solve the offset pedals, transmission tunnel ingress into the footwell and footrest position I'd be sorted 😆