Good Lawyer Guide's response

If you have a fixed term tenancy agreement the rent cannot be increased during the first fixed term – normally six or 12 months. After this, you become a ‘periodic’ tenant unless you sign a new fixed term agreement.

Once you are in a periodic tenancy, the rent can be increased in three possible ways. By the amount set out in the original agreement; by formal agreement between you and the landlord or by the landlord giving you formal notice of his intention to increase the rent. This notice must set out how much it is to increase and when it will take effect. The landlord must give you notice of at least one rental period