MF is right, at the time of review the rent will be brought up to the "current open market rent" which is usually established by a valuer.
However, the tenant is just as entitled to employ a valuer to argue their side of the case.
In this situation a negoation process is started, this can be costly and time consuming and ultimately any rent agreed will be back dated to the rent review date.
Having said all that, deductions can be made from the market rent value when other considerations are taken into account. An example might be that the open market rent is based on new premises, whereas the property in question is, say, 50 years old and a bit run down. This is why you would employ a valuer - to argue for deductions on the current market rent.