My brother has a '10 van that has been running rough, loss of power etc. He returned it to the dealers and they have told him the roughness is due to him using Supermarket diesel (in this case from Tesco's), is this a credible answer or are they just fobbing him off?

I know you've had a lot of stuff about Diesel Particulant Filters and you recommend running the vehicle at over 2000 revs for a period to help them work properly etc, has this any bearing on his problem?

I've recently bought a diesel X5 and I'm obviously wondering about whether to use Supermarket diesel or not!