The momentum is building up for a change in London's housing policy after the election. The ASI published "The Green Noose" by Tom Papworth in January, showing that over a third of protected Green Belt land is devoted to intensive farming, by no means pretty to look at or environmentally friendly, and which in fact generates net environmental costs. In February London First published "The Green Belt - A Place for Londoners?" giving the facts and figures on London's land, and showing that only 26% of London's Green Belt consists of environmentally protected land, parks, and public access land. They similarly showed that only 27.6% of London is covered by buildings, roads, paths and railways.

In today's City AM Mark Boleat, policy chairman at the City of London Corporation, makes similar points, quoting the London First report, and pointing out that "a full 60% of the Green Belt is private agricultural land."

The research done by bodies such as the Adam Smith Institute and London First contradicts the popular image of the Green Belt as green and pleasant land. Far from the daisy-strewn meadows and woods teeming with wildlife that the term suggests, much Green Belt land is farmland, with monoculture fields by no means friendly to wildlife or accessible to people.

The first step in re-evaluation might be to classify Green Belt land into the different types that comprise it. There is genuinely green land, the fields and woods that everyone likes. There is damaged or brownfield land, partly made up of abandoned buildings, gravel pits and the like. And there is farmland, much of which is not environmentally friendly.

The government that takes office after May's election could take the initiative to redress a chronic shortage of housing where it is needed by allowing building to take place on land of types two and three, while leaving the genuinely green land preserved. The opposition will be much diminished if it is understood that only damaged, distressed or intensively farmed land will be affected. And more to the point, the extra houses will bring down the costs of housing and make it available to more people.