Sometimes mistakes happen when drafting a tenancy agreement. A typical question may be:
I advertised a property at £450pcm. The particulars showed this amount and so did the advert. On signing the tenancy, the tenant also paid £450 first months rent and £450 deposit. The following month, £357 rent was paid instead of £450 along with a letter pointing out that the tenancy agreement stated the rent was £357.00pcm and this was all they were going to pay from now on. On investigation, I have found I have made a mistake on the tenancy agreement and the rent does state £357pcm when it should have said £450. The tenancy is for twelve months.
Ideally, agreement should be reached with the tenant(s) and a new tenancy drawn up and signed by all parties reflecting the corrections made.
If the tenant will not agree to make the changes, as a last resort, rectification could be considered through the courts. Crucially, in the example given above, there is a good amount of evidence showing that both parties intended the rent to be £450. In particular, this is evidenced by the tenant paying £450 at the commencement of the tenancy.
Rectification is an equitable remedy by which the court corrects instruments in order to give effect to the real bargain between the parties. Courts of Equity do not rectify contracts; they may and do rectify instruments purporting to have been made in pursuance of the terms of contracts. [Mackenzie v Coulson (1869) L.R. 8 Eq. 368, 375, per James V.C.] Thus what is corrected is not the bargain, but the expression of the bargain.
You can also look at our Slip of Pen Errors article which deals with incorrect dates in notices.
There is a brief summary of rectification here.