My old man deservedly got done for this about 15 years ago.
(only because a disgruntled former employee blabbed however)
They had rather a clever arrangement. The truck had it's usual 300 litre tank on the side half full of normal duty paid diesel, the fridge trailer he was pulling had a 1000 litre belly tank, because you were allowed to use red diesel in the diesel engine to run the fridge trailer.
My old man designed and made a setup which fed the engine directly from the trailer through a custom made suzy line (same colour & thickness as fog light suzy).
The problem of red diesel being returned to the normal tank was solved by having a custom made hidden return loop that returned excess fuel to the low pressure side of the pump and a one way check valve installed back to front in original return.
Dad was caught (fortunately for him only one vehicle) and ended up in court, paid the associated fines etc. He was lucky enough to have time for his other vehichles to have this arrangement removed and the filters replaced several times before they were too inspected.
Some years later he tasked me with nipping down to fuel merchants in my van (Petrol Escort) to get him some cherry for his fridge (yes all above board then/now).
Now I share both my first & last name with my old man. It seems that the merchants must be in touch with the authorities detailing who and quantities purchased as 2 days later I was pulled by C&E and told my vehicle would be dipped for cherry. When I said go ahead they opened the fuel flap (never caught a whiff of the petrol or wondered why it didn't sould like an old tractor) took a sample and was happy with that.
He then proceeded to tell me that they were going to remove the fuel filter and asked me to lift the bonnet. Imagine the blokes surprise when he saw 4 little spark plug leads smiling at him.
Made my day that did!