Petrol - £1.10pl
Diesel - £1.22pl
Assuming 30k, the annual fuel bills at different MPG are as follows:-
Petrol - 30mpg - £4,994.00 - £0.166ppm
Petrol - 35mpg - £4,280.57 - £0.143ppm
Diesel - 40mpg - £4,154.10 - £0.138ppm
Diesel - 45mpg - £3,692.53 - £0.123ppm
Diesel - 50mpg - £3,323.28 - £0.111ppm
Clear savings can be made at this mileage level by going diesel. However, the difference at the higher end of the MPG scale is minimal. Discounting a 48mpg car in favour of a 53mpg on purely financial grounds is madness as you are talking about a saving of under £30 a month. With this sort of mileage the key points are comfort, reliability, and running costs.
Personally I quite liked the ST TDCi until I found out about the reliability of the 2.2.
Discounting the Mondeo ST because it looks like a barge in comparision to the 3 series which is driven exclusively by men under the age of 40 is ridiculous, but discounting it do to it's ability to spring 4 figure bills would be sound.
Don't get too hung up purely on MPG though, while you can save ~£80 a month running a typical four pot diesel (~50mpg) over a typical four pot petrol (~35mpg), this saving is wasted when you consider the potential drepreciation in asset value associated with running a £9k car for 30k a year. You can spend under half that on a decent VAG/Ford/Vauxhall diesel hatch, and such a saving will dwarf the costs you may have to face with a car not under warranty.
A Mondeo or a BMW may not be your favoured choice, and if you want a Fiat Grand Punto instead then that's fine as it's your money, or at least it's your responsibility to pay it back, but don't kid yourself that there are any circumstances in which such a decision could be considered logical.