20(1)An instrument (not itself a lease)—
(a)by which it is agreed that the rent reserved by a lease should be increased, or
(b)which confirms or records any such agreement made otherwise than in writing,
is chargeable with the same duty as if it were a lease in consideration of the additional rent made payable by it.
(2)Sub-paragraph (1) does not apply to an instrument giving effect to provision in the lease for periodic review of the rent reserved by it.