As the landlord you agree the initial rent with the tenant. However, during the first six months the tenants have rights if they consider the rent is above the market rent to refer the rent to the Rent Assessment Committee for review [See Rent Assessment Committees]. This is very rarely done however.
The rent you charge can include a sum to cover the cost of repairs however these costs cannot be passed on to the tenant in the form of a separate service charge.
Note in particular that you cannot seek to pass on to the tenant the cost of any repairs which are your responsibility under section 11 of the Landlord and Tenant Act 1985 or under the gas or similar regulations.