Well that is 10 tankfuls of 97 octane used up so I decided to revert back to 95 octane.
For the 10 tankfuls I averaged 61.4 mpg as my Fuelly page shows.
I did not notice any deterioration in performance today, though the ScanGauge was showing slightly poorer mpg, however, that may have been more to do with the slightly damp start to my journey and the gale force winds (Forth Road Bridge closed to double deckers).
I know that the mpg is going to drop over the next few tankfuls irrespective of whether I put 95 or 97 in. It has already started to drop over the last two tanks, with the onset of the colder, autumnal weather.
I will certainly update this thread as I go on.