Similar tale told about a Cattle Market back home. Customs and Excise turn up near the end of the day to dip Land Rover tanks looking for red diesel. First farmer comes out and starts a long argument with the C&E officers about how they don't need to test his tank. In the mean time everyone else hops into their Landie and depart, the parking area being a triangle of open space between a couple of roads with no fences or hedges there's no hope of delaying them. Finally exasperated C&E get an answer to their repeated question of why they don't need to dip the tank, "This one runs on petrol".
[In the UK fuel for agricultural machinery isn't taxed but mustn't be used in road vehicles, there's a red dye added to trace it]