One of the big problems here is that Diesel cars have been sold on the fact that they offer much better fuel economy, which is true. However, Diesel only used to be 1 or 2 pence a litre dearer than unleaded petrol. With the massive price gap now, it is not more economical to run diesel unless you do over 16,000 miles a year, which very few private users do to be honest.
It is ridiculous that it is now over £6 a gallon. I'm glad I don't pay my firms fuel bill for my van, I drive about 45,000-50,000 miles a year.