Housing Benefit Delays

Tenancy Types and Management

Due to an error on the tenancy agreement drawn up by th letting agent the Housing Benefit Office stopped payment of benefit for my tennant 2 months into the tenancy. A new agreeement was drawn up and the tenant re-submitted their claim. During this time the tenant was living at the property. The Housing Benefit office re-assessed the claim, reduced by £300 per monts and 4 months later began payments but have not backdated the claim.Despite writing to the benefits office I've not had a positve responce. What can I do?