Joe - you are totally wrong in your representation of what David Cameron actually said.
I challenged the DWP over his statement, and they sent me a transcript of it from Hansard:
Prime Minister: “……As I understand it, all parties are committed to reform housing benefit. That was Labour’s commitment before the election. The housing benefit bill is completely out of control. Labour’s own welfare spokesman said last week that, at £20 billion, it was unacceptable and had to change. What we have seen so far, as housing benefit has been reformed and reduced, is that rent levels have come down, so we have stopped ripping off the taxpayer.”
They also provided me with the background to his statement. They said:
"To increase the downward pressure on rents we expect these changes [to benefits] to exert, we have temporarily introduced a new safeguard which will allow local authorities to make direct payments to landlords where it will help the tenant secure or retain a tenancy.
In return for direct payments we would normally expect landlords to reduce or restrain the rent to around the level of the Local Housing Allowance rate that applies to the size of the property.
To date, we have visited around 80 local authorities to monitor the implementation of these reforms. The majority have reported that they have used the new safeguard to help claimants negotiate down rents, and all plan to use it during 2012 as transitional protection runs out.
In cases where we have heard of the safeguard being successfully used, rents have been reduced and the taxpayer has benefited."
So no, David Cameron did not lie to Parliament in the way you say he has. He said that housing benefit had been "reformed and reduced" - he did not say that the total housing benefit bill had been reduced, which is what you say he said.