DSS tenant left owing big arrears, and has been lying to benefits office. Where do we stand?

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We have just suffered a benefits tenant leaving owing £1500 back rent. According to neighbours, her partner has been living there 'for months' and tenant has not informed us as landlords or - to our knowledge - benefits office. They have found a new house privately - leaving us with a dirty house, two old fridges, a manky wardrobe and the usual dirty mattresses in the front garden. Nice.
Trouble is - if I tell housing benefits office, who had been paying us direct while she was in arrears - that her partner had been there all the time, does this mean they will approach us for overpayment? Their letter to us, and the conditions, are not clear.
Apart from that, what should I do to get redress? I know we probably won't see a penny - but would still like to follow the law as far as I can. Thanks for all and any advice.