Lord Neuberger said,

’15. When interpreting a written contract, the court is concerned to identify the intention of the parties by reference to

“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”,

to quote Lord Hoffmann in Chartbrook… And it does so by focussing on the meaning of the relevant words in their documentary, factual and commercial context. That meaning has to be assessed in the light of
(i) the natural and ordinary meaning of the clause,
(ii) any other relevant provisions of the lease,
(iii) the overall purpose of the clause and the lease,
(iv) the facts and circumstances known or assumed by the parties at the time that the document was executed, and
(v) commercial common sense, but
(vi) disregarding subjective evidence of any party’s intentions….

16 .For present purposes, I think it is important to emphasise seven factors.

17. First, the reliance placed in some cases on commercial common sense and surrounding circumstances (eg in Chartbrook… should not be invoked to undervalue the importance of the language of the provision which is to be construed. The exercise of interpreting a provision involves identifying what the parties meant through the eyes of a reasonable reader, and, save perhaps in a very unusual case, that meaning is most obviously to be gleaned from the language of the provision. Unlike commercial common sense and the surrounding circumstances, the parties have control over the language they use in a contract. And, again save perhaps in a very unusual case, the parties must have been specifically focussing on the issue covered by the provision when agreeing the wording of that provision.

18. Secondly, when it comes to considering the centrally relevant words to be interpreted, I accept that the less clear they are, or, to put it another way, the worse their drafting, the more ready the court can properly be to depart from their natural meaning. That is simply the obverse of the sensible proposition that the clearer the natural meaning the more difficult it is to justify departing from it. However, that does not justify the court embarking on an exercise of searching for, let alone constructing, drafting infelicities in order to facilitate a departure from the natural meaning. If there is a specific error in the drafting, it may often have no relevance to the issue of interpretation which the court has to resolve.

19 The third point I should mention is that commercial common sense is not to be invoked retrospectively. The mere fact that a contractual arrangement, if interpreted according to its natural language, has worked out badly, or even disastrously, for one of the parties is not a reason for departing from the natural language. Commercial common sense is only relevant to the extent of how matters would or could have been perceived by the parties, or by reasonable people in the position of the parties, as at the date that the contract was made…

20 Fourthly, while commercial common sense is a very important factor to take into account when interpreting a contract, a court should be very slow to reject the natural meaning of a provision as correct simply because it appears to be a very imprudent term for one of the parties to have agreed, even ignoring the benefit of wisdom of hindsight. The purpose of interpretation is to identify what the parties have agreed, not what the court thinks that they should have agreed. Experience shows that it is by no means unknown for people to enter into arrangements which are ill-advised, even ignoring the benefit of wisdom of hindsight, and it is not the function of a court when interpreting an agreement to relieve a party from the consequences of his imprudence or poor advice. Accordingly, when interpreting a contract a judge should avoid re-writing it in an attempt to assist an unwise party or to penalise an astute party.

21 The fifth point concerns the facts known to the parties. When interpreting a contractual provision, one can only take into account facts or circumstances which existed at the time that the contract was made, and which were known or reasonably available to both parties. Given that a contract is a bilateral, or synallagmatic, arrangement involving both parties, it cannot be right, when interpreting a contractual provision, to take into account a fact or circumstance known only to one of the parties.

22 Sixthly, in some cases, an event subsequently occurs which was plainly not intended or contemplated by the parties, judging from the language of their contract. In such a case, if it is clear what the parties would have intended, the court will give effect to that intention…

23 Seventhly, [he deals with service charges in general]
However, that does not help resolve the sort of issue of interpretation raised in this case.’

[Edited and formatted for ease of reading. Underlining added.] You can find the official law report on Westlaw.