Lord Hoffmann said,

’14 .There is no dispute that the principles on which a contract (or any other instrument or utterance) should be interpreted are those summarised by the House of Lords in Investors Compensation Scheme Ltd v West Bromwich Building Society… It is agreed that the question is what a reasonable person having all the background knowledge which would have been available to the parties would have understood them to be using the language in the contract to mean…

15. It clearly requires a strong case to persuade the court that something must have gone wrong with the language and [judges in lower courts have disagreed] It is, I am afraid, not unusual that an interpretation which does not strike one person as sufficiently irrational to justify a conclusion that there has been a linguistic mistake will seem commercially absurd to another…The subtleties of language are such that no judicial guidelines or statements of principle can prevent it from sometimes happening. It is fortunately rare because most draftsmen of formal documents think about what they are saying and use language with care. But this appears to be an exceptional case in which the drafting was careless and no one noticed.

16. I agree with the dissenting opinion of Lawrence Collins LJ [in this case] because I think that to interpret the definition of ARP in accordance with ordinary rules of syntax makes no commercial sense…

17. … both terms were defined expressions. They might just as well have been algebraic symbols… I strongly suspect that if they had been, they would have made it clear that the parties were intending to give effect to [P’s] construction. But the contract does not use algebraic symbols. It uses labels. The words used as labels are seldom arbitrary. They are usually chosen as a distillation of the meaning or purpose of a concept intended to be more precisely stated in the definition. In such cases the language of the defined expression may help to elucidate ambiguities in the definition or other parts of the agreement… I therefore consider that Lawrence Collins LJ was right to take into account the connotations of contingency to be derived from the defined terms.

19. My Lords, I cannot believe that any rational parties who wished to make provision for such a catastrophic fall in the housing market (itself an unlikely assumption) would have adopted so precise a sum to represent their estimate of what might happen. Why £53,438?… A figure chosen to represent someone’s fears about a possible collapse in the market could only have been based upon wild speculation, not the kind of calculation which produces a figure like £53,438. That figure cannot have been meant to play the part in the calculation which [C’s] construction assigns to it. It must have been intended to function as a minimum land value, not a minimum sale price. To compare it with the realised sale price would not be comparing like with like.

20. It is of course true that the fact that a contract may appear to be unduly favourable to one of the parties is not a sufficient reason for supposing that it does not mean what it says. The reasonable addressee of the instrument has not been privy to the negotiations and cannot tell whether a provision favourable to one side was not in exchange for some concession elsewhere or simply a bad bargain. But the striking feature of this case is.. the provisions as interpreted…make the structure and language of the various provisions of [the Schedule] appear arbitrary and irrational, when it is possible for the concepts employed by the parties …to be combined in a rational way.

21 … I do not think that it is necessary to undertake the exercise of comparing this language with that of the definition in order to see how much use of red ink is involved. When the language used in an instrument gives rise to difficulties of construction, the process of interpretation does not require one to formulate some alternative form of words which approximates as closely as possible to that of the parties. It is to decide what a reasonable person would have understood the parties to have meant by using the language which they did. The fact that the court might have to express that meaning in language quite different from that used by the parties… is no reason for not giving effect to what they appear to have meant.

[He then cites 2 cases on correction]…

25. What is clear from these cases is that there is not, so to speak, a limit to the amount of red ink or verbal rearrangement or correction which the court is allowed. All that is required is that it should be clear that something has gone wrong with the language and that it should be clear what a reasonable person would have understood the parties to have meant. In my opinion, both of these requirements are satisfied.’