I should have updated this months ago so apologies!
I too had the same cracking issue. Had it the first time and changed all 4 not thinking anything of it but when it came back again with fairly low mileage since the fitting (at Audi), I raised it with Dunlop.
It took a long time - around 3-4 months from memory but essentially Audi had to fill in a form per tyre regarding the condition and how much tread was left etc.
They then had to send them to Dunlop but because Audi get them via a supplier, it was essentially Audi to Supplier to Dunlop hence the delay.
Dunlop UK fairly quickly then shipped them to another factory which I believe was in Germany somewhere. Once all 4 were in Germany, I got a report back from the UK office around 2 weeks later to essentially say they were sorry I had problems with the tyre and have advised a dealer allowance per tyre towards the cost of replacing each one.
This based on tread wear.
Tyre 1: 50 %
Tyre 2: 45%
Tyre 3: 55%
Tyre 4: 60%
This equated to % of how much I paid per tyre (roughly) .
Once again, there was then a delay where the tyre supplier got the funds first from Dunlop. This was then passed to Audi and then Audi to me via bank transfer.
This was after around 28-30 days from rec the report.
Given the cost of the 21's, the refund covered 2 new tyres so I still had to shell out for another 2 out of my pocket but I just decided it was best to take what I was given and not use Dunlop again for the A7.
The A3 we have on the other hand were also on Dunlops and had no issues at all with the 17s (I think or maybe 18) taking a fair bit of beating and mileage and so we stuck to Dunlop on the A3.
Might help some of you - the key thing is take loads of pictures. Get the dealer or whoever to also take pictures. Once the tyres are out to Dunlop, you will not see them again so without evidence, its your word against theirs - having said that, the entire process was fairly smooth and no questions asked...just long.
Also bear in mind the buck stops with who supplied (and fitted?) the tyres not Audi but if Audi supplied them, they will deal with their suppliers as you are not allowed to - even though their supplier in my case was someone who I have used many times directly.