Residency Status (immigration) of tenants

Other

Currently letting (via a lettings agent) to a family of 5 including 2 family members who may be in violation of their right to stay in the UK, what documents / proof should be requested to check the due diligence has been done by the lettings agency ? to check that all is in order ? A section 21 has been served and waiting for the courts to resit from September 20th onwards, tenants despite having paid no rent for the last 9 months are for some suspicious reason asking that they be allowed to stay in the property for a further 2 years from now. My gut feeling is this some how links to a need to have a stable address for immigration purposes / applications etc ?? Any comments / welcome. Thank you.