Nice to see you have these sort of constructive conversations on this site ( as a member of loc it might amuse you to know we have members over there who struggle to get 70 miles from £20 in a lexus is200 2.0 six cylinder petrol)
which does not make even your worst figures quoted here too "unlivable" albeit not close to official quoted figures in some cases - all good info here and good reading for someone thinking of sliding over to a mondeo diesel
You will no doubt know the Mondeo and Volvo S40 share the same diesel engines.
Dee, obviously anybody going from a diesel to a petrol engined car will be disappointed in fuel economy. But for my part with my mileage 5.5k, and even taking into consideration dearer Road Tax, but cheaper servicing costs [no DPF etc] running the 1.6 petrol is cheaper. As there is not the diesel preimum to pay at point of purchase. For example with the difference between buying new a 1.6 petrol as against a 1.6 TDCi in cost to purchase. At my mileage it would mean I would have to run the TDCi for 7 years, to save in fuel costs what I paid out extra in the diesel purchase premium. Say for similar spec'ed 2008 on 57 plate second hand cars, it would be 3.5 years of annual savings to pay for the diesel purchase premium.
Obviously it all depends on the annual mileage you do at 10.5K per annum it stacks up at 3.8 and 1.9 years respectively.
At 15.5k per annum it stacks up at 2.6 and 1.3 years respectively to save the diesel purchase premium. For higher annual mileage drivers diesel starts to make sense sooner rather than later.