Over this considerable distance of 600 miles a week diesel should work out cheaper even after paying a premium for a diesel car. But cars under £1,000 are a very dodgy area where one failure could cost more to rectify than the car is worth. There are no 'safe' bets, and beware of cars with a good reputation being sold for more than they are worth. There will almost inevitably be at least one major fault with anything you buy otherwise why would the vendor be selling it? Your best bet is a bereavement sale, or a sale where the owner has had to give up driving which are most likely to be genuine sales.