Last year I tried BP Ultimate 97RON for about 4 weeks followed by BP Ultimate 95RON for the same period. Then I tried Tesco 99RON for the same followed by my old favourite Tesco 95RON.
Results - no difference
My trusty spreadsheet maintained a constant 39MPG throughout the experiment. During this time the trip computer insisted I was doing 44MPG but fuel put in versus actual miles covered proved otherwise.
I do 250 miles a week so I put in about ½ a tank each week which towards week 4 should have removed any 'mixture' of the various fuels.
I've now been running with Tesco95 for about 6 months with no problems.