Only thing I think I can possibly meaningfully add here (being a pedestrian defender owner :p ) is it's one thing to say the extra purchase price diesel V petrol takes 7 years to re-coup. However, when you go to sell the vehicle you will usually get significantly more than the petrol, so you'll probably get a lot of it back.
When oil goes up again the difference might be that you'll still be able to sell the diesel as opposed to giving away the petrol. :o

