i never bother with the expensive diesels, tried them in the past and only difference i noticed was about an extra 3 quid to fill the tank.
So i am now just being a tart and running round checking who's cheapest for diesel, ie Asda etc or the 5p off vouchers from tesco......lol and using Millers additive
With Petrols its a different story. I always used v max, or preferably Tescos' 99 in the Mini. Tescos 99 is better due to the higher ethanol content giving a slightly better octane overall ( and cheaper) . Ont he Mini as it was highly tuned and mapped specifically for 99 fuel it had to be run on it. If on the odd occasion i was stuck at a petrol station with no super i tried it with normal 95 octane and it pinked like a bitch so i backed right off. after that i always carried octane booster in case the same happened again.