It has been said on this site very often. You don't change cars to save money. There are lots of reasons to change a car, but you will rarely save the amount of money it will cost you to change.

You are driving 20,000 miles a year at about 44mpg I work it out to be. Buying a diesel for only £5,000 (an addiitonal £1,500) will get you something that does no more than say 55mpg - lets say 60mpg. So....

20,000 miles at 44mpg = 455 gallons = £2,730pa @ £6 per gallon

20,000 miles at 60mpg = 333 gallons = £2,000pa.

So it will take over two years to get your money back and the car has done 40,000 miles. No point. By the time you have saved £750 the car will have done 60,000 miles and depreciated like a stone. Do bother changing cars for economy sake.