my vote also goes for Tesco. when i was running my evo 8 (400hp) as a daily i had all sorts of issues for a while which after about 500 quid turned out to be fuel related - after many frequent tank dips/tests after fill ups at both shell and tesco, it turned out v-power wasnt quite what it said it was.......(car was pretty borderline mapped to eek power) thereafter i stuck with tesco 99, no problem whatsoever...
at the end of the day i guess engines /maps could act differently in answer to fuels?, since its all the same stuff initially until they add all the proprietry additives, especially in the higher octane options at the pump