Notwithstanding anything in the definition of occupier in subsection (1) of section 379 of the Act of 1947, lands and heritages shall not be deemed for the purposes of section 243 of that Act (which relates to unoccupied subjects) to be occupied as respects the year 1963-64 or any subsequent year by reason only that they are subject to a tenancy or sub-tenancy; and the said section 243 shall have effect as respects any such year as if the word " unlet", wherever it occurs in the said section, were omitted.