I have a friend, who brought a diesel engined van, which had been run on "chip fat" used chip fat at that, it stunk, not only inside, but driving behind it also.
I do know, that, the ministry regulary check LGV vehicles at weigh bridges, and dip everyone,
Should you be found to be running on illegal diesel, your vehicle gets impounded, they work out how long you have owned it, calculate the mileage, from previous keeper, then assume you have always been cheating the system, so you owe them all that calculated back tax,
Usually more than the vehicles worth, unless its a Tractor Unit.
Not worth it, in my humble opinion
Oh the chip fat van took a new set of injectors, and a right thrashing to rid itself of it