Magnets - don't work and never will. You'd wonder why the auto industry spends billions of pounds a year on efficiency if it could be solved by a £2 magnet. The only effect they have is psychological - so save yourself some money and just be lighter with your right foot.
Petrol - supermarket petrol comes from exactly the same refineries as BP, Shell, Esso or any other oil companies. Its not a cheap version of petrol other than the price and if anyone states any different they are talking crap. In fact, due to the high turnover, it could actually be of better quality than some that has been sitting in a tank of a quiet village station for some time. Quality and maintenance of the tanks are key to the quality but it's always the same stuff that goes into them whether it's Esso, Shell or a supermarket (apart from the already mentioned points of additives)