“Now, I have said that the terms and meanings of the original contract were fixed in March/April 2003. The same analysis can be made of any subsequent variation to add or omit. The extent of any such variation was fixed at the time it was made. I say this because it is Mr Marsden's submission that the subsequent conduct of the parties can be used as an aid in the construction of the original contract and/or any variation thereof. Reference was made to the treatment in Chitty on Contracts of Whitworth Street Estates, cited above. I gave counsel the opportunity to put in further written submissions on that case. Miss Gough took the opportunity. I have since re-read the report. Mr Marsden says that Whitworth Street Estates applies only to written contracts. I respectfully disagree. At page 603 Lord Reid says this: ‘It has been assumed in the course of this case that it is proper, in determining what was the proper law, to have regard to actings of the parties after their contract had been made. Of course the actings of the parties (including any words which they used) may be sufficient to show that they made a new contract. If they made no agreement originally as to the proper law, such actings may show that they made an agreement about that at a later stage. Or if they did make such an agreement originally such actings may show that they later agreed to alter it. But with regard to actings of the parties between the date of the original contract and the date of Mr Underwood’s appointment I did not understand it to be argued that they were sufficient to establish any new contract, and I think they clearly were not. As I understood him, counsel sought to use those actings to show that there was an agreement when the original contract was made that the proper law of that contract was to be the law of England. I must say that I had thought that it is now well settled that it is not legitimate to use as an aid in the construction of the contract anything which the parties said or did after it was made. Otherwise one might have the result that a contract meant one thing the day it was signed, but by reason of subsequent events meant something different a month or a year later’. And […] Lord Wilberforce dealt with the point in this way: ‘In my opinion, once it was seen that the parties had made no express choice of law, the correct course was to ascertain from all relevant contemporary circumstances including, but not limited to, what the parties said or did at the time, what intention ought to be imputed to them on the formation of the contract. Unless it were to found an estoppel or a subsequent agreement, I do not think that subsequent conduct can be relevant to this question’.”
“The logic of that applies equally, in my view, to any oral or part oral contract. Once the words used to form the contract/variation have been established they fall to be construed in the light of the circumstances at that point in time”
“If I am wrong in my interpretation of Whitworth Street Estates I think there is force in Miss Gough’s observation that subsequent conduct, here, could prove a double-edged sword for the defendant”
“The court may not look at the subsequent conduct of the parties to interpret a written agreement. However, where the agreement is partly written and partly oral, subsequent conduct may be examined for the purpose of determining what were the full terms of the contract”
“Where the contract is only partly written, nothing in the authorities prevents the court from looking at the way the parties acted for the purpose of ascertaining what terms were agreed but not written down. Thus in Wilson v Maynard Shipbuilding Consultants AG Limited the Court of Appeal held that where one cannot ascertain from the terms of the contract itself what was agreed about a relevant term, in that case the place where under his contract an employee normally works, one may look at what has happened and what the parties have done under the contract during the whole contemplated period of the contract for the limited purpose of ascertaining what that term is.”