Answer - about 100 metres. Found this out yesterday evening after putting £60 of diesel in the tank. Felt a right tw@t pushing it back to the garage!!!!
Never (yet!) done the diesel thing but did take the tank out once, put it back then drove off down the road without refilling! Must have gone a good 2 or 300 yards (that's metres to you kids) before realising, three point turn and made it home on the fuel line and filter