to be honest the way i look at supermarket fuel is:

1) my car definately performs better on shell, even 95ron shell compared to tesco 99
2) i've never heard of anyone being disapointed with shell
3) tesco gets mixed reviews

therefore just going on the reviews my conclusion is shells better! pluss shell is always no dearer or cheaper than the supermarket stuff and i live near shell so i just fill up with the good stuff 8)