What is the test for restricting the words in the exclusionary clause in a contract?

In Glynn v. Margetson & Co. their Lordships went to the length of restricting the words of the relevant clause itself to make it consistent with the main purpose of the particular contract before it. Here I am simply applying the exclusionary clause to the portion of the transaction to which the parties agreed it should apply. I am satisfied that the more important the term or the breach, the clearer it must be that the clause was intended to exclude it: see Lord Wilberforce in the Suisse Atlantique case, at p. 432.