Looking around, it would cost me an extra £1600 to get the diesel over the petrol, that's 2 1/2 years to see my money back from saving in fuel and tax. If I trade the car in in say 5 years at 90,000 miles I'll be £1600 better off plus any retained value. That said, it relies on nothing big going pop in that time....
Really not sure what to do here... I can see the benefits of diesel, but if it goes wrong it carries a price tag...