<Dangerous Suggestion>
Fill a can with Super, and a can with ordinary. Spill a small puddle of each on concrete, and light!
The Super will burn noticeably differently, a lot more uniformly, and faster. Whether your engine is bothered though I could not say.
</Dangerous Suggestion>
As for fuel, my local Shell usually matches the Tesco in Watford and can be the cheapest non supermarket fuel in the country. But it a bit of a bun fight unless you fill up at weird times. I do, but living 5 minutes away, hardly a problem.
