Londynek

A fixed term agreement is for a fixed period agreed in the rental agreement (usually six months or one year). A tenancy could be fixed term even if rent is paid each week/month. If the fixed term ends and no new tenancy agreement is drawn up then the tenancy agreement becomes periodic.
A periodic agreement rolls from week to week, or month to month. These include tenancies that were originally for a fixed term that have ended and no new fixed term agreement has been drawn up.

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Ending a periodic tenancy agreement
If the agreement is periodic (rolling from week to week or month to month), a tenant will normally have to give at least four weeks' notice to end it, or a calendar month if it is a monthly tenancy. The notice must be in writing and must end on the first or last day of the tenancy, unless the tenancy agreement allows it to be ended on a different day. If rent is paid less frequently a tenant has to give at least one rental period of notice. So if rent is paid every two months, two months notice would be required.

A tenant may choose to end a periodic tenancy be issuing a valid notice to quit to the landlord. Once the notice expires then the tenant’s agreement will have ended.