Tenants living in a property that should have been licensed, but has not, can apply to the Residential Property Tribunal Service to claim back the rent they have paid during the unlicensed period. They can claim back up to 12 months rent.
Councils can also claim back any Housing Benefit paid to landlords in the same way.
The Residential Property Tribunal Service adjudicates on these matters. Their web-site can be found by following this link: Residential Property Tribunal Service