(1)The terms of a secure tenancy may be varied in the following ways, and not otherwise—
(a)by agreement between the landlord and the tenant;
(b)to the extent that the variation relates to rent or to payments in respect of rates or services, by the landlord or the tenant in accordance with a provision in the lease or agreement creating the tenancy, or in an agreement varying it;
(c)in accordance with section 103 (notice of variation of periodic tenancy).
(2)References in this section and section 103 to variation include addition and deletion; and for the purposes of this section the conversion of a monthly tenancy into a weekly tenancy, or a weekly tenancy into a monthly tenancy, is a variation of a term of the tenancy, but a variation of the premises let under a tenancy is not.
(3)This section and section 103 do not apply to a term of a tenancy which—
(a)is implied by an enactment, or
(b)may be varied under section 93 of the [1977 c. 42.] Rent Act 1977 (housing association and other tenancies: increase of rent without notice to quit).
(4)This section and section 103 apply in relation to the terms of a periodic tenancy arising by virtue of section 86 (periodic tenancy arising on termination of a fixed term) as they would have applied to the terms of the first tenancy mentioned in that section had that tenancy been a periodic tenancy.