Since all tenancies are now periodic under the Renters' Rights Act, rent increases can only happen through the Section 13 process. Your landlord must serve you with a formal notice on the prescribed form (Form 4A), giving you at least two months' notice of the proposed increase. Rent can only be increased once per year.
When you receive a Section 13 notice, you have three options: accept the increase, negotiate with your landlord, or refer the notice to the First-tier Tribunal. The tribunal will assess whether the proposed rent is in line with the market rate for comparable properties in your area. If the tribunal determines the increase is above market rate, it will set a lower figure.
Your landlord can't increase your rent by any other method. A clause in your tenancy agreement that allows for rent increases outside the Section 13 process is unenforceable for assured periodic tenancies.