For more than 700 years the Tillingbourne river has produced power on the spot where Neil and Candace McMahon's converted mill house now stands. A southern wall of Old Netley Mill, which dates from 1860 when this part of Surrey produced much of Britain's gunpowder, still shows distinctly where the water wheel once turned.

But the house has not generated energy since the 1970s when a modern pumping station was built elsewhere and the wheel dismantled. Two years ago the McMahons set out to change that. The pair make unlikely green crusaders. Both have doctorates in geology and have worked in the oil industry for more than a decade. Nevertheless, they are among an increasing number of Britons clambering aboard the government-driven renewables bandwagon.