I bought a 3 year old A3 last March from an Audi dealer and it came with the usual Audi Approved 1 year warranty.
I have now received an invitation to take out a further years cover. I was looking at the T&C's and came across this;
"The continued validity of your cover is dependent upon you having the covered
vehicle serviced by your Audi Centre or any other Audi authorised repairer in
accordance with the manufacturer’s specifications"
So presumably anyone with an Audi extended warranty can't use an independent garage for servicing. I realise this doesn't apply to manufacturers new car warranties.