Historically, Customs & Excise are part of The Treasury and are essentially Revenue Collectors. They are not particularly interested in the driver, moreso the Operator.
What many people fail to realize is that virtually all large vehicle are now fitted with a fuel-flow meter and a computer memory capable of storing a year worth of data. (Possibly even more with the latest vehicles). Two, maybe three, years ago I went to a presentation where one of the engine manufacturers was explaining what information could be downloaded to a lap-top in terms of fuel usage, rev usage, gear changes, idle time, brake applications, etc. The person commented that with the latest systems, they were still developing the software to download the data but, “It didn’t matter because the memory was big enough to hold a year’s worth.”
If C&E find a fuel marker, they will simply impound the vehicle, download the fuel data, note the odometer reading, request the fuel receipts, and then present a bill based on what THEY calculate to have not been paid.
If you don’t pay the bill, they sell the vehicle, and still come after the Operator for anything outstanding.
The odd accidental ‘squirt’ of Red into the wrong tank, although embarrassing, is not the end of the world. In fact, during the fuel protests (was it really 4 years ago), certain firms were invited to run on Red should their Derv supplies dry up in order to ensure that foodstuffs reached the supermarket shelves.
As I’ve said, the vehicle will provide data indicating how much fuel has passed through the engine, and if the receipts do not tally with the quantity used by a particular vehicle, then where has it some from?