reduction in housing benefit

Housing Benefit Overpayment

I have taken on a new tenant with the HB being paid direct to me, and the HB has been reduced by £10.20 per week, after the first month. I was sent a letter stating the invoice number but the HB department won't send me the invoice or the reason behind the deduction.
The overpayment was from my tenants previous address. I have written to HB who say they have every right to take money direct from by payments. This has now put the tenant in serious rent arrears as to secure the direct payment I did take advanced rent. My understanding is that they could only claim overpayment from me if I was actually paid the overpayment or was aware of changes to the tenants circumstances. Any help wold be apprieciated