Ok...so...this debate had me intrigued as to whether or not I had been wrong in my thinking and my judgement clouded so I decided to try and conduct my own little test and here's what I found.
I ran my tank to 5 miles from empty to try and get my supermarket fuel as low as possible. I then filled the tank up to fuel using Shell V Power (98 RON) ahead of driving to St Andrews and back on a mini break and back with mrs Rizzle. This is a distance of around 175 miles there and back travelling, for the most part, on national speed limit dual carriageways and national speed limit B roads. What I quickly noticed was an improvement in the responsiveness of the car and in the fuel consumption which leapt up from my disconcertingly low 32 mpg (still need to get this checked out) to figures up in the 45mpg range. Now, I generally drive at the speed limit plus 10% (sorry to those that that offends) on 70mph roads. I was hugely impressed by the predicted remaining range when we returned. Since returning I have been driving a little on B roads but, for the most part, in and across the city where I tend to drive in 30/40/50mph zones and in the usual traffic conditions for a city. I have subsequently also watched my fuel consumption fall...and fall...and fall (as to be expected).
I have filled up again to a second full of V Power and my fuel consumption seems to have settled at around the 34.4/8mpg mark. This is still an improvement in miles returned by tank and I do still feel the car is a little more responsive than it was with supermarket fuel. However, I haven't yet done the maths to work out if that's actually meant that it's more economical to use this fuel despite its significantly higher cost. However, I suspect not particularly with supermarkets announcing and introducing price cuts from 2-3p per litre. I think (and this is very approximate) that I'd need to see a return of 4-5 miles greater per tank for V Power (or similar 98 RON fuel) to make financial sense. I would say though that the car definitely does feel better/more responsive using a higher grade fuel...but is that enough when fuel costs such a premium and there is a war on fuel being waged?!
*Disclaimer - this was written on my phone and, if I'm honest, I can't be bothered proof reading it all so please ignore any predictive text errors etc*