Keep a record of contacts made, exchanges, etc., and the relevant correspondence.
We have had problems with a flat we own. At different times my daughters have moved in after we got rid of a bad tenants.
For the first, employees of a debt collection agency rocked up one evening, but providing the agency with a copy of the tenancy agreement proved my daughter was not the person they were looking for.
For the other daughter, we had some initial contact from HMRC concerning the previous tenant which was sorted very quickly. However, daughter two has had recently had parcels delivered to the address which are not in her name. Also a mobile telephone contract on the same name. The mobile company resolved it with one telephone call (sorry) and plod have become involved.