25p - 35p per litre sounds about right as a mark-up. Mind you, we paid over £2 per litre recently at Swansea, and I am pretty sure the mark-up is more at some places.
You don't need underground storage, a mobile bowser will do the trick, but that is an investment. Actually some fuel companies provide the bowser if you buy enough fuel from them, and the more you purchase, the bigger the discount (as normally a delivery charge is charged which will be the same for 5000 litres or 10,000 litres).