The answer seems to be ‘no’ provided that the written contract contains wording similar to ‘all variations to this Licence must be agreed, set out in writing and signed on behalf of both parties before they take effect’. This is called a ‘no oral modification’ clause.
There are good reasons for using no oral modification clauses in that they prevent attempts to undermine written agreements by informal means.
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If parties wish to change a contract, it is important for them to follow the formal procedures set out in the contract to vary its terms. If an agreement contains a no oral modification clause, an oral agreement to change the contract will not be effective, even if all the parties agree to the change.
Rock Advertising Ltd v MWB Business Exchange Centres Ltd  UKSC 24, 16 May 2018
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