Open market rent review involves a periodic revaluation of the rent based on what the rent would be if the premises were re-let fresh at the date of review (eg would the rent have increased if a new lease was created?).
The new rental amount is based on rents in the local market for similar leases ('comparable premises'). If after the review, it's found that the rents in the local area have increased then the rent for the premises can be increased.
In most situations, the landlord and tenant should try and agree the new rent themselves.
Normally, both parties appoint their own property surveyors to help manage the process and negotiations.
The use of the premises can have a significant effect on the open market rental value. If the lease allows the tenant to use the premises for a variety of purposes, the open market rental value will be calculated as if they are being used for their most valuable purpose.
For example, if office rents are higher than rents for storage space and the lease allows the the use of the premises to be either offices or storage, the rent will be calculated as if the premises were offices.