With Regulated tenancy the landlord or tenant can object to the rent assessed by the Rent Service.
If so the case will be passed to a Rent Assessment Committee to decide the Fair rent. Tenants with assured tenancies may also apply to the RAC for a determination of the open market rent they should be paying.
They may also apply if the landlord serves notice of a proposed rent increase. The rents they set will be the legal maximum that can be charged for one year from the date of the decisions, after which an application for an increase can be made. Relevant forms are available from the Rent Service website