". A little further afield, in Bradford for instance, there are swathes of derelict industrial land and massive empty mills surrounded by boarded up shops, empty health centres and closed schools. Why is this? Dare I say that the land is not profitable enough for the landowners to sell to housing companies yet? That they are sitting on areas perfect for urban redevelopment until they can make more money out of it?

Just my 2p"

Have you considered the fact that brownfield sites are likely contaminated, and that can extend to underlying aquifers. They require investigation, evaluation and probably remediation before any construction can occur. Greenfield sites present far less of a headache. Old gas works, chemical works and decommissioned power stations offer unique expensive remediation problems and any sort of 'made ground' can create subsidence problems. Having spent a period of time on such projects I know the solutions can come with a hefty price tag, and without government funding, it ain't going to happen.