i read a report about tesco's fuel
the reason why it is cheap is cos it is the left overs from the refineries and it has less additives to keep the octane rating up
the main stream fuel from shell esso etc comes from near enough the same place as tesco's but their's have more additives to ensure better performance
as soon as the fuel leaves refineries it slowly starts to lose octane rating so it always helps to know when your local petrol station receives it's deliveries
also it is a good idea to leave your tank near enough empty when u go on holz as the fuel has lost a lot of its octane rating