For anybody interested this turned into a SAGA with a unhappy ending. As there has been no resolution and given the treatment, I am now in a position to name and shame.
After initially emailing Audi UK, they advised I visit my local parts department at my nearest Audi dealer which happens to be literally 2 miles away. Bristol Audi, part of the Mon Motors Group. I was quite upfront with Audi that the only sensible conclusion would be a free replacement.
The parts department, although helpful said that I should go through Service, so I got moved to the service department. It was quite obvious the person who spoke to me wanted to go on their lunch break, so probably just wanted to shoo me on. They claimed that someone must have tried to steal my mirror glass which is apparently very common (that well known crime obviously!) and that it wouldn't be covered by anybody. So was quoted £190 to fit and replace!
Always useful to remember that on the A6, the mirrors fold in when the car is locked...
Anyway, I didn't want to cause a scene at the dealership despite being quite frankly speechless at the fact that no-one even looked at my car and was just dismissed with a rubbish reason for the failure. It's a £40k+ vehicle that has had a mirror glass fall off - something I didn't even know was a thing! So I rang Audi UK - the person I spoke to said that I had a really strong claim, especially as no one even looked at the vehicle and was quite shocked at how dismissive they was - so far so good.
Two weeks passed, no response.
Finally, somebody rang me - but it turns out my claim hadn't been uploaded to the system properly, so had to go through it all again. The person inputting it clearly didn't understand what my problem was and seemed to be getting confused with the fact that I was amazed that a dealership I had literally just booked my car in to have the cambelt replaced etc at a cost in excess of £1k with the mirror glass issue. Anyway, she said it would be investigated and would get back to me.
48 hours. Investigation complete - the dealership, Bristol Audi claimed I never visited and that it is essentially their word against mine.
I am not an angry person, but I was absolutely gobsmacked! I told them they could look at the CCTV and see my car driving into the dealership (it was a flippant comment, but they pointed out they can't do that) - I was fuming tbh. They said they would go back to Bristol Audi and do further investigation.
Another 48 hours passed. They rang again, apparently parts do remember speaking to me, but service didn't. Funny that. Anyway, i've had enough - i've been waiting a month for a new mirror glass and i'm just going to go to an independent and do it myself. This isn't about the money, you don't buy an Audi for cheap car ownership, but you do expect exemplary customer service and for simple parts of your car to not fall apart a 5 years old.
I've cancelled by cambelt service etc at Bristol Audi and would rather a 90 mile round trip to my next nearest one that isn't owned by Mon Motors Group.
What a palava - I have to be honest, it's put me off buying another Audi going forward. Not that anyone will care, but i've never known such shoddy customer relations - especially from what I consider a reasonably premium brand.
I would be interested to know how old your car is as mine did exactly the same thing about 2 weeks ago. Mine is a late 2016
