Update on this problem
Took the car to an independent Audi specialist today.
He looked at the car and inspected all the suspension components. Said he couldn't find any leaks. Also said there was no fault codes coming up on VCDS. He did mention though that it might be seals in the compressor.
He did suggest that I took the car to Glasgow Audi to have the suspension system pressure tested. I'm very loath to do this after a previous bad experience with them.
Should I go to another Audi specialist in Glasgow area for a second opinion?