Is supermarket fuel quality being compromised due to the large mark up they are being allowed to charge?

My 25k mile Audi Q7 engine light came on recently and turned out it was a Nox sensor failure, Strange thing is whilst in the Audi dealer I was told it would take a few days to get the part as there was a run on failures so parts were low. Only a week later my brother rang to say same fault on his Audi Q5. Now these parts aren't cheap, £800+, fortunately my car is under warranty.

Is it possible to prove what caused the damage and how can this be avoided in future?