JT wrote:
Well I have an LPG tank in my back garden, and consume about 3,000 litres a year to heat the house. Now if my consumption were to double then yes it might seem a bit suspicious, but in reality who would do anything about it? I know for a fact that a larger and less insulated house down the road has a gas bill approximately three times mine, so the actual numbers wouldn't trigger any alarm bells unless the increase was overnight. But even if it was, who would tell Customs and Excise about it? Can you see the gas supplier complaining that they are "selling too much gas"?
Secondly, who would actually have a record that I had a gas powered vehicle?
As regards the mechanics of filling it, you could easily tap into the gas feed to the house in an unobtrusive way in order to extract the fuel.
In fact, even if you were to do so blatantly, what could anyone do about it? I can't see it being illegal simply to own the correct gear for filling your car, surely they'd have to catch you in the act.
Which would be damned near impossible if your filling routine took place in a windowless garage, which had a perfectly legitimate gas point fitted for a wall heater...
Going by what ziltro has said, it sounds as if it's all a bit ad hoc, and that they don't really expect to be encountering massive fraud on this. That said, I'm sure if it did catch on, they'd start being a lot more aggressive about it - and, of all the government departments, HMCE are the ones with the legal clout to be the most aggressive.
What intrigues me somewhat is what technical means they might possibly be able to deploy to control it. Some kind of special "tagged" additive for veg oil or gas, along the lines of the red dye in diesel, only the other way around? Only that wouldn't work, since conventional diesel doesn't have such a tag, and they'd need expensive equipment to detect it.
(I'm aware that we're probably fairly wildly off-topic on this forum with this...*grovelling apologies to the moderators*

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