An exclusive supply agreement contained a clause that said that it could only be amended by a written document signed by both parties. But was that clause really effective to stop the parties from varying the agreement by an oral agreement or by conduct?
Although, on the particular issues in this case, it was not necessary to decide the point, the Court of Appeal said that it was not. In principle, under English law, parties can agree whatever terms they wish (subject to public policy limits), whether in a document, orally, or by conduct. Consequently a clause such as that in the supply agreement did not prevent the parties from later making a new contract varying the original contract by an oral agreement, or by conduct. Difficulties of proof may, of course, arise but the facts then have to be determined by the court from the evidence.