Due to being asked a previous similar question, I asked a friend with a doctorate and also a degree in Chemical Engineering/ currently working at a major UK refinery to find out answers to some such questions and this is the reply, summarised by me (I'm neither a chemical engineer, an engineer, a scientist or a mechanic and therefore it is summarised in my words, in good faith and any disagreement with the content, I won’t challenge):
* There is a difference between branded and supermarket fuel.
** The refinery produces fuel which meets all of the necessary specifications/regulations.
*** Then, when, for example a major branded tanker arrives at the refinery, in addition to loading up with the fuel, they add their own secret mix of additives to the fuel. These additives differ between the branded companies and is guarded as to the ingredients. The additives are designed to clean the engine as it runs so as to prevent problems over a long period of time. Apparently, the additive really does work and is not a gimmick.
**** Supermarket fuel, on the other hand, does not necessarily come from a local refinery. Apparently, it can arrive from far afield including mainland Europe, if they can get it at a better price. It will in any event meet all of the necessary specifications/requirements; however will not necessarily contain the additives. It does not damage your car it just means that, for example, valves could get more deposit build ups over a long period of time.
***** The other issue is the difference between basic and enhanced fuels, as one example Shell Vpower. This is considered to be very good indeed as; for example, the petrol burns cleaner and might well result in a smoother acceleration/ better performance experience from the car. It is a purer fuel. You should not notice any difference in performance in an ‘ordinary car’, but in performance cars it does make a difference. However, as it burns cleaner, it helps keep your engine cleaner and so, even for an ordinary car, it is considered to be worth putting a tankful of branded enhanced fuel in now and again.
So, that info, along with my previous logic and what I have learnt from threads like this means that reference so called 'normal cars', I will continue to fuel with supermarket fuel (using time to time tanks of enhanced branded fuel); & for so called performance engines, I will continue to use enhanced branded fuel.