Customs has a "Writ of Assistance', which is effectively a balnket search warrent that allows them access to pretty much anything, anytime (although at night the Police need to be present). They don't normally use it, prefering instead to get search warrents. However, they can and will use it where necessary.
They can also get a conviction based on rebated fuel in the fuel filter, even if the tank is drained. If you google the UK duties tribunal and then search for excise rebated fuel, you will see a lot of cases that Customs won, and almost none that the vehicle owner won
If you are using red diesel, stop it as purchase records give them the lead. If you are thinking about it, don't bother