Housing benefit overpayment not my fault

[Guest]

My housing benefit payments take into account my state pension and small BT Pension.* BT informed me there has been an accounting error in my pension and I have been underpaid by £10.55 a month.* I immediately informed housing benefit and they are making me pay all this back from when my pension started 5 years ago. I have appealed of course but they say even tho it's not my fault and I could not have possibly known of this I still have to pay it back.* They increased my rent by £44 a month, to Pay this back and have knocked that down to £20 a month.* Is this correct legally?