I was as surprised as you are.
Method was:
I collected the vehicle from seller on 12th April and filled the tank from empty to the brim with the supreme fuel which cost me £107 Approx. I recorded the mileage (km) at the filling station.
When the tank was between a third and a half full I topped it up (to the brim again) with 33.2 litres of the low grade fuel. (The km had gone up by 268km) Therefore indicating I had used 33.2 litres of the E5 since filling up the first time.
I then used the second batch of fuel (which would have been a mix of the two - roughly 60% E10 to 40% E5)
I then filled up the vehicle a third time, this time when the light came on the vehicle took 60.9 litres to refill and I had driven a further 272km since fill up number 2.
I was very surprised by the results, I did use the air con slightly more in the second period and I was driving more frequent short journeys as opposed to the first time.
I drive minibuses frequently as part of my job and have been trained to be very light footed on the gas/brakes. My style of driving could be described as a snail on Valium.
This being said I really noticed the difference between the two fuels, the vehicle purred and drove beautifully on the supreme, it felt like acceleration was slightly more erratic and the engine felt like it wasn’t as smooth on the E10.
I have now refuelled with E5 and will repeat the process. I am unsure if Esso in Cornwall sell 97 Octane or 99 Octane fuel but whatever it was it ran beautifully.
This third time I filled up with 97 Octane E5 from Texaco. Unfortunately the closest Esso from where I live is 25 miles away. I will definitely fill up there again if I am in that area though.