In brief, our research throws up the following conclusions:
There's no scientific evidence to suggest that supermarket fuel is inferior to its brand-name equivalent.
There are some proven benefits to using more expensive "super" fuels, but these benefits are only really pronounced in sporty vehicles.
Third-party fuel additives don't seem to stand up to their bold claims when tested. However, some people state additives work well for them. You could try them for yourself to see if they make a difference, as they cost just £5 a bottle so may be worth trying - if only once!