I usually use Tesco for fuel. The one sample that failed was a one-off purchase (Somerfield??). I would never query it with a supplier -the effect I would expect would be for them to ban me from filling jerrycans. They don't know it's for an aeroplane.
Apart from that one instance, I usually do one check only.
Mariner's proper method is interesting. I suspect the right proportions
petrol:water might be O.K.
I'll mention a proper measuring cylinder to the group.
That 2.5% alcohol is acceptable is interesting.