Lord Wilberforce said,
‘It is often said that, in order to be admissible in aid of construction, these extrinsic facts must be within the knowledge of both parties to the contract, but this requirement should not be stated in too narrow a sense. When one speaks of the intention of the parties to the contract, one is speaking objectively — the parties cannot themselves give direct evidence of what their intention was — and what must be ascertained is what is to be taken as the intention which reasonable people would have had if placed in the situation of the parties. Similarly when one is speaking of aim, or object, or commercial purpose, one is speaking objectively of what reasonable persons would have in mind in the situation of the parties. It is in this sense and not in the sense of constructive notice or of estopping fact that judges are found using words like “knew or must be taken to have known” (see, for example, the well-known judgment of Brett L. J. in Lewis v. Great Western Railway Co. (1877) 3 Q.B.D. 195 .
I think that [the previous cases cited] are saying, in different words, the same thing — what the court must do must be to place itself in thought in the same factual matrix as that in which the parties were. All of these opinions seem to me implicitly to recognise that, in the search for the relevant background, there may be facts which form part of the circumstances in which the parties contract in which one, or both, may take no particular interest, their minds being addressed to or concentrated on other facts so that if asked they would assert that they did not have these facts in the forefront of their mind, but that will not prevent those facts from forming part of an objective setting in which the contract is to be construed. I shall show that this is so in the present case.’
[Edited for ease of reading. You can find the full report on Westlaw].