Another possibility might be to lift the side of the fitting opposite to that where the bolt and its nut are rotating. The now angled top fitting then puts extra pressure on the nut which may be sufficient to hold it steady while the bolt is unscrewed. If you are lucky this will do the trick, but remember to keep the pressure on as the screw is undone. Keeping the pressure on may require that you put a wedge such as the tip of a flat screwdriver under the top fitting and in the immediate vicinity of the bolt you are trying to remove, thereby keeping the pressure on until eventually the bolt separates from the nut. Good luck when you come to refitting the bolt, as that nut will still be loose unless you have managed to secure it. If there's one thing that diesel oil is good at finding, it's somewhere to leak through, and a leaking fuel tank even at the top of the tank is something that you really will want to avoid.
