Reg 104. Sums to be deducted in calculating recoverable overpayments.
Basically once you have created an overpayment – you allow any amount of Housing benefit that should have been determined had the true circumstances of the claimant been known at the time.
The CAB seem to have got the wrong end of the stick – underlying entitlement is there to reduce the overpayment – it is not entitlement to Housing benefit and can only be applied to the period of the overpayment.
You are not going to pay them the underlying entitlement – because they have already received benefit for that period – but at the wrong rate.
What has been done is not a revision and is not arrears of Benefit (which in some cases can be used wholly to reduce an overpayment).
I have answered this in a very basic fashion, but am sure that others will quote the necessary if that is required.