We were doing some work for National Grid in Anglesey a few years ago, and I was filling a tractor up at a local petrol station with white diesel. (This was a highly unusual procedure from the perspective of the garage owner - red diesel only for tractors surely!).
Customs and excise were staying in the same hotel as us, and had several marked vehicles on the car park each evening. This came up in conversation with said garage owner who told me that they were checking ‘weights and measures’ on the island, and had been to spot-check the calibration of his pumps the week before. ‘Must have been a bit of a worrying event for you?’ I suggested. It turned out he was actually pretty upset as his pumps had been under-reading!
In relation to this thread - measure oil into 20l Jerry can(s), fill with 20l at petrol station. Decant into combi-cans as required. Has never caused a single issue in 25 years.