For obvious reasons, the cheapest cars to run in the UK tend to be diminutive, lightweight city cars but the Dacia Sandero is a fully-fledged supermini and a diesel one at that. The petrol powered Sandero Access is almost as cheap to run overall, but we'd recommend you go for the 1.5 diesel in Ambiance trim - it's better equipped, easier to keep up with traffic and costs about £65 a year less than the 1.2 to run.
Total cost of ownership (3 years, 36,000 miles): £7,139.70 (Petrol: 7,204.49)
Monthly running costs: £198.33
Cost per mile: 24p