“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[1998] 1 WLR 896 , 912-913. They are well known and need not be repeated. 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.”