I had the problem with my previous A5 and first dealer said they would need to run the diagnostic first at £120, and didn't get any reassurance that they would help with the cost if I got lumped with a £1200 build.
Rang another local dealer from a different group, they explained that if I was their customer they wouldn't make me pay, said they would run a diagnosis for free, and if it was the regulator, they would only make me pay the £120 diagnosis fee towards the fix. Ended up getting brakes and gearbox service done while it was in so it ended up only costing me £100.
Shop around matey!