My bet would be on a faulty hazard switch. Id recommend a diagnostic check to see if any fault codes flag up.
This is quite common on the C5 Dave, and it's most often caused by the hazard switch failing as I had similar problems with my C5.
You'll need a diagnostic check to see if any fault codes flag up.
It's probably the roof clamps that has failed. Problem if you can't get into the boot as you could manually control the boot from there.
Had a look online and found this:
https://www.audiworld.com/forums/a6-s6-c6-platform-discussion-58/stuck-tow-hitch-2970418/
https://www.google.com/amp/s/www.audi-sport.net/xf/threads/211403/%3famp=1
https://www.audiworld.com/forums/a6-s6-c6-platform-discussion-58/oem-trailer-hitch-stuck-2931386/
You're most welcome. You need to make sure you get one from the same engine and gearbox as yours as the undertrays can vary depending on the engine or gearbox due to there differing in size etc.
It might not be as bad as you think as the under tray is in two parts. Front for the engine and rear for the gearbox. Looks like it's come apart at the join.
Id recommend you get a second opinion from another Audi dealership as well as from an Audi independent specialist.
Then contact the motoring ombudsman. You'll be waiting 6 months for a reply though.
You need a diagnostic check to see if any fault codes flag up.
Also what engine has the car got?
Does it happen in certain weather or temperature conditions?
Any warning lights on?
Any recent other work done to the car?