The most economical answer is a front-wheel drive diesel-engined car a few years old that does not have a diesel particulate filter, fitted with winter tyres through the winter. They are more effective than four-wheel drive with summer tyres, though, of course, a 4WD fitted with winter tyres is even more effective.

A 2-year-old Honda CR-V 2.2iCDTI does not have a DPF, does have four wheel drive (but only when it needs it) and the one I had averaged 41mpg over six months, which is about as good as it gets for that size of 4WD. So a CR-V 2.2iCDTI fitted with winter tyres over the winter. Smaller, a Toyota Urban Cruiser 1.4 D-4D is the lowest CO2 4WD available under £20,000, narrowly beating the FIAT Panda 4x4 diesel. The MINI Countryman All4 diesel is the lowest CO2 4WD available over £20,000.