The common law requires the amount of rent to be certain.
The amount is normally stated in the lease.
In the case of commercial premises is usually subject to regular review.
Following any such review the new agreed rent should be recorded in a rent review memorandum signed by the parties. If the rent has merely been agreed, for example, between surveyors, it may not be enforceable if there is uncertainty caused by a lack of documentary evidence.
The lease will normally provide when the new rent (i.e. after a review) is to take effect. If the rent has been agreed by the review date then it will become due from that date. If it has not been agreed by the review date then:
The parties should examine the wording of the lease for guidance. For example, it may provide that the tenant must continue to pay rent at the old rate, with any excess that becomes due once the rent has been agreed to be paid on the quarter day following completion of the review. The lease may also provide for interest on that excess, but if the lease is silent there is no automatic right to interest.
If the lease does not make provision, it may be arguable that no rent need be paid until the new rent has been ascertained. This may, however, be a risky position for the tenant to take, and in the absence of direct authority it would be wise to continue to pay the rent at the existing rate.
Where the lease is silent, the tenant's obligation to pay the new rent will take effect from the quarter day following completion of the review: South Tottenham Land Securities Ltd. v R. & A. Millett (Shops) Ltd  1 WLR 710.
Where the rent review is decided at arbitration the review is completed when the award is made and published to the parties, i.e. when the arbitrator informs the parties that his award is available: The Archipelagon  2 Lloyd's Rep 289.
An agreement by an assignee with the landlord to pay the reviewed rent by stepped increases will bind the original tenant, therefore, if the assignee defaults, the landlord may pursue the original tenant for payment: GUS Property Management Ltd v Texas Homecare Ltd  27 EG 130.
Where the lease is assigned by the tenant between quarter days, the Apportionment Act will apply. The assignor will be liable to pay rent up to the date of the assignment, and the assignee thereafter. However, the landlord can seek to forfeit for all the rent due following the assignment, notwithstanding that the assignee is only personally responsible for the post-assignment rent. Parry v Robinson-Wylie Ltd (1987) 54 P+CR 187.
Overpayment of the rent by mistake is recoverable by the tenant: Nordin & Peacock Plc v D. B. Ramsden & Co Ltd (No 2) Butterworths case reports 5.2.99 (applying Kleinworth Benson v Lincoln City Council  4 All ER 513). The tenant's time to reclaim the overpayment only begins to run when he becomes aware of the mistake: Limitation Act 1980 s. 32(1)(c).
Underpayment of rent due to the landlord's miscalculation allows the landlord to still recover that underpayment: University Superannuation Scheme Ltd. v Marks & Spencer Plc  4 EG 158.
If a tenant gives notice to terminate the tenancy and then fails to vacate, the landlord will be entitled to twice the rent that would otherwise be due: Distress for Rent Act 1737, s.18.