Evidence of prior negotiations and proposals cannot be used to construe a written contract where the effect would be to contradict the plain language of the contract, even where the language is ambiguous; such evidence reveals the terms the parties intended or hoped to make, which are superseded by and merged in the contract itself. The practical benefit doctrine from Williams v Roffey Bros does not apply to a variation agreement where the evidence does not identify any benefit the promisee would obtain beyond existing contractual entitlements, and there is no promise by the promisor to do anything additional. The question whether ambiguity is a threshold requirement for recourse to surrounding circumstances evidence under Codelfa remains unresolved.
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