With some flats in central London costing more than a private island, its small wonder housing is considered the number one issue facing the capital.

All the candidates have fallen over each other promising ever more homes as a result. There’s little detail on how they’ll be delivered, just the usual vague promises to release more public land or tweak the planning system.

The simple truth is the Mayor of London doesn’t have the power to solve London’s housing crisis alone. Many of the factors pushing up demand – low interest rates, public subsidy, increased household formation – are beyond their control.

But while the Mayor can’t in reality make the housing crisis much better, they could definitely make it a lot worse. And sadly in Sadiq Khan and Zac Goldsmith, the two leading contenders, exactly that will happen.

It’s hard to see who will be worse: Khan wants the power to freeze rents – and even Swedish socialist economists will tell you rent controls don’t work – while Goldsmith previously described himself as a ‘proud Nimby’.

Khan thinks 50 per cent of all new developments should be affordable housing, which would render many unviable, and Goldsmith has a pathological aversion to anything tall.

In an effort to put ‘Londoners first’, both demonise international buyers. The Panama Papers have raised some (justified) questions about foreign ownership of London property, but global investors are crucial to financing development activity and shouldn’t all be tarred with the same brush.

Admittedly there are a few areas they do get right: welcoming further institutional investment into the rental market stands out. Their joint support for additional transport infrastructure is also positive. Crossrail has helped unlock new opportunities for developers and no doubt Crossrail 2 will do the same.

But a serious mayoral housing strategy would focus on working with central government and the surrounding counties.

While the politics of planning is often local, the legislative framework remains national, and still largely rooted in the 1947 Town and Country Planning Act. Fundamentally, this needs replacing with a North American or European-style rules-based system, which makes far greater use of zoning and is presumptive in favour of development. Reform on this scale can only come from Westminster.

The worst legacy of the 1947 Town and Country Planning Act is the greenbelt. An arbitrarily defined no-go zone for developers, it has acted as a constraint on growth for London and critically, nearby commuter towns. This is why a joined up approach to housing with neighbouring local authorities is needed.

The leafy Shires around London need convincing overhauling the greenbelt doesn’t mean urban sprawl. The case for action is clear: if you want the city you so rely on (as does the rest of the country) to remain internationally competitive, it needs homes for its workers. And many of those homes will have to be built on the greenbelt.

Collaboration between London and adjoining authorities could go much further, with the pooling of funds to attract large-scale investors and bankroll development as Greater Manchester’s councils have done.

Manchester and other regional cities might offer another way out of the mess too. The base reason for London’s affordability issues is the sheer volume of people who want to live here. But turning them away would be economic self-harm. So the solutions must be supply-side (building more homes to house them all).

Yet if the Northern Powerhouse and Midlands Engine emerge as genuine competitors, and don’t just stay buzzwords, the flow of young people and migrants looking for decent jobs and that metropolitan buzz could start to shift elsewhere.

In short: to fix London’s housing crisis, think beyond London. It’s a shame Sadiq and Zac won’t.