Chris - you say "I've clearly stated that last year was the first where housing subsidy was negative" and you seem to think that this is due to a change in the rate of "investment" (i.e. reduction in "subsidy").
The Local Government Accounts report 2011 where the figures are presented clearly states that the drop is the result of interest payments on the HRA.
Furthermore, the significant change in the figures in 2004/5 came from a change in the accounting practice for the HRA, not from a change in subsidy:
From table 651 of the accounts: "From 2004-05, rent rebates and Government subsidy for rent rebates are no longer part of Housing Revenue Account subsidy".
According to "Rent Rebate Subsidy Limitation: Derogations 2010-2011, England" (DCLG):
"The Local Government Act 2003 moved accountability for rent rebates from the Housing Revenue Account to the General Fund"
So the rent subsidy is still there, it just does not appear in the Housing Revenue Account.