Housing benefit I'm being taken to court, help

[Sea Air]

I have a flat that I rented out 18 months ago, I took on a Tennant who was claiming housing benefit and the council were paying it direct to me, everything was OK until January (2018)this year when they stopped paying me, the reason being they said my Tennant moved out August 2017 and are claiming theoverpayment amount paid to me for that time back and threatening court action. My Tennant however hadn't moved out and was still occupying my flat until mid March when his tenancy expired. Where do I stand, I don't regard this as my debt and accepted the payments for rent, how do I prove he hadn't moved out last summer, all I have is a handful of text and Facebook messages from him stating that he hadn't told them he moved and was still in my flat and proposed move out plans for March