Variations to original tenancies.

Tenancy Types and Management

I have just bought a block of flats with some rather old but definitely ASTs tenants in residence. During the history of this block there have been some changes to the agreements but they have not been recorded in writing but remain agreed by the tenants.

The changes include the amount of rent, the frequency of payments, if payable in advance or arrears and what services the landlord provides.

I appreciate I can use a section 13 notice to change the future situation. But my question is what is the lawfulness and enforceability of the present changes that are not documented but are agreed between the parties.

Obviously I wish to avoid new tenancies.