I suspected a local garage of this and reported my suspicions to trading standards. They checked it out and assured me all is well, although I don’t fuel up there any more. If you have suspicions about a specific site then I suggest you try Trading Standards first.
What Hammer said.
The pumps have to be calibrated in line with the Weights and Measures Act. Trading Standards also ensure the garage is complying with it’s licence to sell petrol, which was why the garage I worked at years ago was reminded more than once that we had an attended service licence. 
