Housin Association...Alledged Rent Arrears

I lived in a KHT (knowsley Housing Trust) property fo 24 years ... my rent was payed by Housing Benefit for the whole of that time . Due to ill health i was admitted to Hospital on 04/05/14. I was in Hospital till i was discharged on 09/06/14... Just before my stay in Hospital i decided to give up the property & move to a place that was better suited ... I informed ALL the authorities (KHT, Housing Benefit, DSS, Health services) etc. i was discharged from Hospital to my new address on 09/06/14.... The new tenancy started on 06/06/14... I am now being persued by Rossendales collect for £613.16 in rent arrears ? KHT say i never informed them about leaving the property . i informed them at the same time i informed everybody else ... They say i owe the arrears because the housing Benefit stopped even though they paid rent for a months notice of leaving the premesis. Rossendales collect are sending me intimidating letters saying i will be getting a visit from their Debt collectors in a few days ... KHT have totally ignored my dispute to this Debt & it is causing me great concern & i'm sick with worry . could you give me some advice please ? Thank You...

Housin Association...Alledged Rent Arrears