As from the end of the period of the statutory tenancy, the operation of the Rent Acts in relation to any premises which during that period were, or formed part of, the dwelling-house shall be the same as if neither the premises which were during that period the dwelling-house nor any part of those premises had ever been let before the end of that period:
Provided that nothing in the foregoing provisions of this section shall affect any part of the premises so long as a person retains possession of that part by virtue of the Rent Acts in consequence of a letting effected during the period of the statutory tenancy.