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For your kind of miles a petrol engine is probably the one to go for. Diesel engines have generally been more complex than petrol ones (turbocharging systems, EGR valves etc) up until very recently, and if you're looking at a V50 then anything other than a T5 is a 'standard' non turbo petrol. Less to go wrong, and not substantially enough extra fuel consumption to affect you at your kind of mileage, especially given the higher cost of diesel fuel.

Relative cost/performance benefits aside, with stricter emissions testing in the MOT set to come into force and the possibility of new regulations targeting diesel cars, I wouldn't want to risk being left holding the keys of a previous-generation used diesel car that could become worthless, or in the worst case scenario unroadworthy or banned from certain cities, overnight.