Having dealt with contractual interpretation, Lord Hoffmann then turns to justifying the exclusion of evidence of negotiations.
’32. Critics of the rule, such as Thomas J in New Zealand ( Yoshimoto v Canterbury Golf International Ltd  1 NZLR 523 , 538–549) Professor David McLauchlan (“Contract Interpretation: What is it About?” (2009) 31:5 Sydney Law Review 5–51) and Lord Nicholls of Birkenhead (“My Kingdom for a Horse: The Meaning of Words” (2005) 121 LQR 577 –591) point out that although all this may usually be true, in some cases it will not. Among the dirt of aspirations, proposals and counterproposals there may gleam the gold of a genuine consensus on some aspect of the transaction expressed in terms which would influence an objective observer in construing the language used by the parties in their final agreement. Why should the court deny itself the assistance of this material in deciding what the parties must be taken to have meant?…
33.…The general rule, as I said in Bank of Credit and Commerce International SA v Ali  1 AC 251 , 269, is that there are no conceptual limits to what can properly be regarded as background. Prima facie, therefore, the negotiations are potentially relevant background. They may be inadmissible simply because they are irrelevant to the question which the court has to decide, namely, what the parties would reasonably be taken to have meant by the language which they finally adopted to express their agreement… that will usually be the case. But not always. In exceptional casesa rule that prior negotiations are always inadmissible will prevent the court from giving effect to what a reasonable man in the position of the parties would have taken them to have meant. Of course judges may disagree over whether in a particular case such evidence is helpful or not. In Yoshimoto [the judge] thought he had found gold in the negotiations but the Privy Council said it was only dirt. As I have said, there is nothing unusual or surprising about such differences of opinion. In principle, however, I would accept that previous negotiations may be relevant.
34. It therefore follows that while it is true that…inadmissibility is normally based in irrelevance, there will be cases in which it can be justified only on pragmatic grounds. I must consider these grounds…
35. The first is that the admission of pre-contractual negotiations would create greater uncertainty of outcome in disputes over interpretation and add to the cost of advice, litigation or arbitration…As against this, it is said that when a dispute over construction is litigated, evidence of the pre-contractual negotiations is almost invariably tendered in support of an alternative claim for rectification… or some alleged exception to the exclusionary rule. Even if such an alternative claim does not succeed, the judge will have read and possibly been influenced by the evidence. The rule therefore achieves little in saving costs and its abolition would restore some intellectual honesty to the judicial approach to interpretation.
36. There is certainly a view in the profession that the less one has to resort to any form of background in aid of interpretation, the better. The document should so far as possible speak for itself.
As Popham CJ said in the Countess of Rutland’s Case (1604) 5 Co Rep 25b , 26a:
“it would be inconvenient, that matters in writing made by advice and on consideration, and which finally import the certain truth of the agreement of the parties should be controlled by averment of the parties to be proved by the uncertain testimony of slippery memory.”
37. I do not think that these opinions can be dismissed as merely based upon the fallacy that words have inherent or “available” meanings, rather than being used by people to express meanings, although some of the arguments advanced in support might suggest this. It reflects what may be a sound practical intuition that the law of contract is an institution designed to enforce promises with a high degree of predictability and that the more one allows conventional meanings or syntax to be displaced by inferences drawn from background, the less predictable the outcome is likely to be.
In this respect, it is interesting to consider the reaction to the statement of principle in Investors Compensation Scheme Ltd… v West Bromwich Building Society…, which was viewed with alarm by some distinguished commercial lawyers as having greatly increased the quantity of background material which courts or arbitrators would be invited to consider…The only points it decided that might have been thought in the least controversial were,
first, that it was not necessary to find an “ambiguity” before one could have any regard to background and,
secondly, that the meaning which the parties would reasonably be taken to have intended could be given effect despite the fact that it was not, according to conventional usage, an “available” meaning of the words or syntax which they had actually used.
38. … I rather doubt whether the ICS case produced a dramatic increase in the amount of material produced by way of background for the purposes of contractual interpretation. But pre-contractual negotiations seem to me capable of raising practical questions different from those created by other forms of background.
Whereas the surrounding circumstances are, by definition, objective facts, which will usually be uncontroversial, statements in the course of pre-contractual negotiations will be drenched in subjectivity and may, if oral, be very much in dispute. It is often not easy to distinguish between those statements which (if they were made at all) merely reflect the aspirations of one or other of the parties and those which embody at least a provisional consensus which may throw light on the meaning of the contract which was eventually concluded.
But the imprecision of the line between negotiation and provisional agreement is the very reason why in every case of dispute over interpretation, one or other of the parties is likely to require a court or arbitrator to take the course of negotiations into account. Your Lordships’ experience in the analogous case of … Pepper v Hart  AC 593 suggests that such evidence will be produced in any case in which there is the remotest chance that it may be accepted and that even these cases will be only the tip of a mountain of discarded but expensive investigation…
39. [He then looks at French law, concluding] English law…mixes up the ascertainment of intention with the rules of law by depersonalising the contracting parties and asking, not what their intentions actually were, but what a reasonable outside observer would have taken them to be. One cannot in my opinion simply transpose rules based on one philosophy of contractual interpretation to another, or assume that the practical effect of admitting such evidence under the English system of civil procedure will be the same as that under a continental system…
41. The conclusion I would reach is that there is no clearly established case for departing from the exclusionary rule. The rule may well mean, as Lord Nicholls has argued, that parties are sometimes held bound by a contract in terms which, upon a full investigation of the course of negotiations, a reasonable observer would not have taken them to have intended. But a system which sometimes allows this to happen may be justified in the more general interest of economy and predictability in obtaining advice and adjudicating disputes. It is, after all, usually possible to avoid surprises by carefully reading the documents before signing them and there are the safety nets of rectification and estoppel by convention…
42. The rule excludes evidence of what was said or done during the course of negotiating the agreement for the purpose of drawing inferences about what the contract meant. It does not exclude the use of such evidence for other purposes: for example, to establish that a fact which may be relevant as background was known to the parties, or to support a claim for rectification or estoppel. These are not exceptions to the rule. They operate outside it.’
[Edited and formatted for ease of reading. You can find the original law report on Westlaw.]