Unless you regularly do 20,000 miles a year, or need to tow something, or carry heavy loads or carry seven people, don't buy a diesel. Get a small turbo petrol or a hybrid. Generally, Peugeot/Citroen 1.2 Puretech are the best because they combine power, torque, fuel economy and (so far) excellent reliability. Puretech 130 best; Puretech 100 next best. All do about 50mpg, but automatics 5 - 10mpg less.