Details are a bit sparce here?.

Your current car is to big and thirsty, but you don't say what it is or what MPG you get, so how can we advise something smaller and more efficient?.

What is the smallest size of car you could manage with, or want?. Also, you currently have a 4x4, does it's replacement need to be a 4x4/SUV, or are you happy to get a conventional car?.

You only account for 5 out of the 7 days in a week regarding mileage, what is the annual mileage likely to be?.

Without some clarity it is difficult to make a reccomendation, but, going by what you have mentioned, there seems to be no case for diesel. At that budget you will be looking at a more modern complex diesel engine, but it will be a good age, and have had quite a few owners by now. Extra maintenence costs (unless doing it yourself) is more than likely to offset the extra fuel cost of a simpler N/A petrol.

AT that price range, for what you want you wont go far wrong with the Toyota Avensis. I have had three and never spent apenny or repairs, just normal servicing.

Or a Prius, or an Auris hybrid. Despote the complexity, these are very reliable, and will be more economical than your 4x4, whatever make that may be.