That's exactly why i recently bought a petrol Jeep Patriot as our family car. The diesel was £3000 more for only an extra 10mpg (30 for the petrol combines, 40 for the diesel). When you take into account the 20% more that diesel costs as well it's a no brainer.
Unless of course, you buy your cars as the diesel will certainately retain more value. But nobody is crazy enough to buy a Jeep (i hope) expecially when they do a £207pm, no deposit, 12,000 miles per annum, hand it back at the end deal. That's £7452 over 3 years (plus 2 years tax as year 1 is included) on a car that probably depreciated by that as soon as i drove it from the forecourt. It's got a GFV of £10,600 which it will never be worth in 3 years - no wonder these big car makers need to borrow billions if i can see that the deals don't make sense.