I have a bit of a thing with tesco fuel i cant conclusivley prove it but heres what evidence i have so far :
just lately (this will be the fifth french car comming in with same problem) a customer brought in a pug 206 gti 180 and said the antipollution light keeps comming on, we went through everything as usual couldnt find anything apart from a slightly over dirty fuel filter for the mileage. changed it gave the engine a thorough clean drained existing fuel and went and put £10 of shell v power in, took it for a 15 mile test drive, everything was fine no problems.
so signed it off and customer was happy.
3 days later came back with same problem this time we checked everything from fuel system to sensors nothing doing, anyway one of the rac guys came in with a car and we had a chat, he said "I bet you its that crappy tesco petrol, my mates a tanker delivery driver and they use the worst grade posible".
so i phone the customer back and after a good chat he thinks this could have been happening after filling up at tesco alot lately, and the light comming on seems to be about the same time, so 3 days later he calls back in and says its never happened since ive been filling up at morrisons
. like i say this is the 5th or 6th time we had this problem and couldnt resolve it, i cant say anything about the diesel, but maybe something to think about.