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Rock Advertising Limited v MWB Business Exchange Centres Limited concerned just such a non-oral modification clause. Could it really work? Surely, a merchant’s freedom to vary his affairs in a way the common law decrees effective is paramount. Can such a provision really double-entrench itself?

The Court of Appeal thought not. A fellow can agree whatever he chooses, however he chooses – in writing, orally or by conduct. Following that general principle, a “no oral modification” clause (a “NOM” clause) would not prevent him later making a new oral contract to vary the original contract.

Good, that’s that all sorted and we can now all move on to more important issues of the d —

BUT WAIT. The Supreme Court has disagreed. Lord Sumption (for it was he) dismissed this “fallacious” reasoning: a chap’s autonomy operates until he has made his contract; thereafter only as far as the original contract allows.

Quoth milord: “The real offence against party autonomy is the suggestion that they cannot bind themselves as to the form of any variation, even if that is what they have agreed.”

But what if contracting parties have relied on an oral variation in good faith, and by conduct abided by it for a good period? As have so many of his brother judges in the past, here Lord Sumption looked lovingly towards the courts of chancery in a clean-handed defendant’s aid. A wronged party might seek to argue an equitable estoppel. However, the scope of this estoppel will be limited:

“... at the very least, there would have to be some words or conduct unequivocally representing that the variation was valid notwithstanding its informality and something more would be required for this purpose than the informal promise itself.”

Common sense has taken a bit of a battering, but this is all good news for we learned wordwrights who can now be prayed upon to paper otherwise unnecessary amendment agreements for merchants to vouchsafe their obvious commercial intent.