In a relevant lease (i.e. in a lease of land granted before 1 September 1974 for a period of not less than one hundred and seventy-five years), any provision which stipulates for payment of a casualty is void; and, accordingly, any such payment which would, but for this subsection, have fallen due on or after the relevant day shall not be exigible. "Casualty" means any duplicand or other periodical or casual payment stipulated in a relevant lease to be payable by the tenant to the landlord in ad