[Zebster]

This has nothing to do with filters (DPF or otherwise).

Did the dealer confirm that ALL available updates (physical and software) have been applied to your car?

The specific 'EGR' thread I previously linked indicated that dealers can be far more helpful than your experiences here. I recommend that you post in that thread where your problem will be seen by others who have been successful in this regard.

Blaming supermarket fuel is a poor excuse and I'm not sure why they suspect 'engine valve leakage'. As before, I suggest a second opinion.