I found this link useful when comparing the cost of petrol vs. diesel cars
http://www.parkers.co.uk/cars/petrol-vs-diesel/?deriv=43253#manu=9909&model=1549&deriv=43253
I've set it for the Leon FR and it compares the diesel against the petrol. In the case of the Leon because the petrol is more expensive to buy new, its a no brainer.
That's pretty cool, but only really applies when buying brand new doesn't it?
I can buy an 08 Diesel for £13,500 or an 08 Petrol for about £12,500 so it doesn't seem to really work out.... it would take 24 months for the Diesel to have paid for itself (the extra £1000 i paid for it for example) but the problem is the MPG figures they give are very optimistic (38 for the petrol and 53 for the Derv).