Sadly mate, evidence it is not, you would need to sample the fuel with the seller present, seal it in front of them, label-sign and seal with a tamper proof seal which is recorded on the paper work then send it for testing with people signing for it along the way.
Ive had a fair bit of training on the collection of evidence, its extremely painful to collect evidence that will hold up legally.
Its like they say in defense, is your arse isn't covered, its gonna get screwed.