Where a mortgage clause contains an admitted mistake and neither party seeks rectification, the court will give the mortgage efficacy by construing it as securing the minimum amount that can safely be concluded was intended. The surety construction principle (construing guarantees in favour of the surety) applies to determine the extent of a surety's liability, not merely to determine whether the mortgagor has the character of a surety. The distinction between permissible background facts and impermissible evidence of actual intentions under Codelfa remains difficult to draw in practice.
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