In ascertaining for the purposes of this Act the duration of a lease of land the following provisions shall have effect.

Where the terms of the lease include provision for the determination of the lease by notice given by the landlord, the lease shall not be treated as granted for a term longer than one ending at the earliest date on which it could be determined by notice given by the landlord.

Where any of the terms of the lease (whether relating to forfeiture or to any other matter) or any other circumstances render it unlikely that the lease will continue beyond a date falling before the expiration of the term of the lease, the lease shall not be treated as having been granted for a term longer than one ending on that date.

Sub-paragraph (3) applies in particular where the lease provides for the rent to go up after a given date, or for the tenant’s obligations to become in any other respect more onerous after a given date, but includes provision for the determination of the lease on that date by notice given by the tenant, and those provisions render it unlikely that the lease will continue beyond that date.

Where the terms of the lease include provision for the extension of the lease beyond a given date by notice given by the tenant this paragraph shall apply as if the term of the lease extended for as long as it could be extended by the tenant, but subject to any right of the landlord by notice to determine the lease.

It is hereby declared that the question what is the duration of a lease is to be decided, in relation to the grant or any disposal of the lease, by reference to the facts which were known or ascertainable at the time when the lease was acquired or created.