Lord Wilberforce said,

‘In order for the agreement…, to be understood, it must be placed in its context. The time has long passed when agreements, even those under seal, were isolated from the matrix of facts in which they were set and interpreted purely on internal linguistic considerations. There is no need to appeal here to any modern, anti-literal, tendencies, for Lord Blackburn’s well-known judgment in River Wear Commissioners v. Adamson (1877) 2 App.Cas. 743 , 763 provides ample warrant for a liberal approach. We must, as he said, inquire beyond the language and see what the circumstances were with reference to which the words were used, and the object, appearing from those circumstances, which the person using them had in view. Moreover, at any rate since 1859…it has been clear enough that evidence of mutually known facts may be admitted to identify the meaning of a descriptive term.

…On principle, the matter is worth pursuing a little, because the present case illustrates very well the disadvantages and danger of departing from established doctrine and the virtue of the latter. There were prolonged negotiations between solicitors, with exchanges of draft clauses,…
The reason for not admitting evidence of these exchanges is not a technical one or even mainly one of convenience, (though the attempt to admit it did greatly prolong the case and add to its expense). It is simply that such evidence is unhelpful. By the nature of things, where negotiations are difficult, the parties’ positions, with each passing letter, are changing and until the final agreement, though converging, still divergent. It is only the final document which records a consensus. If the previous documents use different expressions, how does construction of those expressions, itself a doubtful process, help on the construction of the contractual words?… it may be a matter of degree, or of judgment, how far one interpretation, or another, gives effect to a common intention: the parties, indeed, may be pursuing that intention with differing emphasis, and hoping to achieve it to an extent which may differ, and in different ways. The words used may, and often do, represent a formula which means different things to each side, yet may be accepted because that is the only way to get “agreement” and in the hope that disputes will not arise. The only course then can be to try to ascertain the “natural” meaning. Far more, and indeed totally, dangerous is it to admit evidence of one party’s objective — even if this is known to the other party. However strongly pursued this may be, the other party may only be willing to give it partial recognition, and in a world of give and take, men often have to be satisfied with less than they want. So, again, it would be a matter of speculation how far the common intention was that the particular objective should be realised.

In my opinion, then, evidence of negotiations, or of the parties’ intentions…ought not to be received, and evidence should be restricted to evidence of the factual background known to the parties at or before the date of the contract, including evidence of the “genesis” and objectively the “aim” of the transaction.’

[Edited for ease of reading. You will find the original report on Westlaw.]