There is no evidence of any increase in land hoarding, and developers do it so that they can maintain a constant level of work (ie they can't afford to sack everyone if they happen t have no land to develop).
Developers are not interested in maintaining a 'constant level of work', though, are they? They are interested in maintaining a constant level of profit, and if they think they will make more profit from restricting supply, they will restrict supply. They will not willingly build if their profits will be reduced. That is stating the obvious.
Housing should not be seen as a market, because it is not. It is rigged, like all other areas of life in the benighted UK, and rigged to benefit one section of society over another.
Criticism of these policies tend to come from the point of view that a real free market currently exists, that it is nearly perfect, and that it will all be ruined if anyone tries to tinker in any way at all. And they tend to come from those who benefit from the status quo - and from billions in public money that goes in subsidy to private landlords.
We used to have a much different housing sector, which has been 'reformed' to the point of our current crisis. We didn't used to need housing benefit of any sort. We had controls on rent instead, and no massive subsidy to the sector. We need 'counter-reform' and Miliband has made a start, even if only in changing perceptions about the inevitability of our current wasteful and bad system of housing the citizens.