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. It’s the petrol powered Sandero Access that is cheaper to run overall, but we'd reccommend you go for the 1.5 diesel instead, in Amiance trim - it's better equipped, easier to keep up with traffic and only costs about £6 a year more than the 1.2 to run.
Total cost of ownership (3 years, 36,000 miles): £8,687.57 (Diesel: £8,230.06)
Monthly running costs: ££228.47
Cost per mile: 23p