Interestingly, and on the subject of Tesco (again) I understand from a contact in the refining business that:
"EN228 (Petrol) allows for 5% ethanol and most Tesco petrol has ethanol in it as does all fuel in France."
This would enhance the performance of standard petrol by raising the octane level.
On diesel: "Fuel specs - EN590 (diesel) allows 5% biodiesel to be blended in (without you needing to know). The biodiesel component gets a 20p/l duty reduction so at 5% it is 1p/l cheaper before blending costs (about 2p/l) so hence why Tesco Gloabl diesel at 1p/l more expensive. Rapeseed is the main diesel crop"
So the premium diesel product at Tesco would appear to be 5% BIO.
There was a school of thought that said that Tesco fuel products were either the same as everyone elses, or, that it was at best, a weak brew. The more we look into this matter it would seem that the opposite is true and that Tesco's products are superior to some of the major oil companies.