That's the issue though - getting a clear diagnosis from Audi, unless the car has totally failed and been towed in. If it's intermittent or failing, rather than failed, you will have to be patient, while the dealer, afraid of not being reimbursed, jumps through hoops with your car to satisfy Audi UK.
It *should* be straightforward to apply the 7 year warranty, but dealer actions say it isnt.