Well it depends on the MPG you are getting at the moment.
I do between 3-400 miles a week
My car is a rover 420SLi which i got 30mpg out of
at say 350 miles a week thats 18200 miles a year. which is 606.6 gallon @ 4.546 lires a gallon that is 2758 Litres @ £103p a litre = £2840.64p
In a dieles that does say 45MPG that would be 404.44 gallon / 1838.60 Litres and @ 109p per litre = £2004 per year in fuel cost.
so on those rought figures I will save around £840 per year. So it would depend on the reasons for changing car on top of the reduction in fuel cost over the period you are intending to have new car.
Personally As my Rover is getting on and things are starting to go wrong I need a new car as repairs bills are beginning to get a tad expensive, so again its then looking at petrol V diesel in the car you are looking for.
So again for Eample take a Mondeo how much would say a 2.0TDCi be compared to a 2.0 petrol over same age and same condition ? then factor in how many years yer planning on keeping it and you can come up with some figures on whether it is in yer interests or not to go Diesel
hope that helps.
EDIt Just checking Autotrader and the difference between a 90k mile 2.0lx Mondeo and a 90k 2.0Td Ghia X ( sorry i dont know whether the specs are equivalent or performance etc) the diesel being a 02 and the petrol a 51 is about £200
so in this instance imho it would seriously be worthwile looking at the diesel as the fuels cost saving easily covers the extra cost in terms of purchase price, then the longer you own it the more you will save.