He was born in Croydon in 1907, the eldest of two children, both sons, of William (a local solicitor) and Christine Diplock. His father was a local solicitor. He was educated at Whitgift, a local independent school, and then at University College, Oxford, where he studied chemistry. Diplock showed little sign of intellectual distinction, and graduated with a second class degree. But active participation in the Union (he was Secretary in 1929) was an early indication of a love of debate. He won a scholarship from Middle Temple in 1929, and performed more creditably in the Bar Examinations than at Oxford: Diplock was the only first class finalist in the Trinity Examinations of 1931, and was awarded a Certificate of Honour. (It was a tough exam: only about half of the entrants passed.) He was called to the Bar by Middle Temple in 1932. Diplock served pupillage with Theo Mathew, the elder son of James Charles Mathew, the founder of the Commercial Court. Quintin Hogg, the future Lord Chancellor Hailsham, was a fellow-pupil. It is unlikely that either of them acquired an interest in commercial law from Theo, who was a libel specialist by then. Diplock next spent time with Valentine Holmes, a thriving junior barrister with a general common law practice. Holmes was junior of choice to Sir Leslie Scott KC, who had briefly been Solicitor-General in Lloyd George's coalition government. Diplock became a member of Scott's chambers in the mid-'thirties, presumably through the Holmes link. Commercial litigation was a significant part of Scott's practice. He had begun his career specialising in maritime litigation in Liverpool, and appeared in around seventy cases reported in Lloyd's Reports.

However, while being in Scott's chambers must have given Diplock a grounding in commercial law, he was not an overnight success. Scott was made a Lord Justice of Appeal in 1935, depriving Diplock of a potential source of work, and Diplock's name did not appear in the law reports until 1937 (in a local government case). His first two appearances in Lloyd's Law Reports came in 1939. (In Sunley v Cunard White Star (1939) 64 Lloyd's Rep 272, a carriage of goods case in which he was instructed for the shipowner, Diplock demonstrated intellectual and moral integrity by withdrawing a plea of fraud which he had concluded was not sustainable. Counsel for cargo interests told the Judge that this was "completely spontaneous and voluntary, and in no way moved by any approach from the plaintiffs".) Also in 1939, Diplock was appointed secretary to Sir Wilfrid Greene, Master of the Rolls. Historically, the secretary had sorted out appeal documents, organised hearing dates, and drafted orders. Exactly what the job involved in Diplock's time is not clear, but it seems unlikely that a busy and successful barrister would have had time for it.

Kenneth Diplock (left) at the Bar point-to-point in 1962. His companion is fellow judicial horse-enthusiast, Lord Oaksey. Formerly Lord Justice Geoffrey Lawrence, Oaksey had been the senior British Judge at the Nuremburg War Trials. He was the son of A.T. Lawrence, who had briefly served as a Commercial Judge after the Great War.

Phliip Cunliffe-Lister, 1st Viscount Swinton, headed the secretive internal security organisation for which Diplock worked during World War II.

Intelligence work did not take up all of Diplock's time. He retained a semblance of a Court practice, with around half a dozen cases in the law reports between 1941 and 1945. It was largely government war-work, for example prosecuting a farmer who had ignored instructions under emergency regulations in Ministry of Agriculture v Price [1941] 2 KB 116 (he was led by the Attorney-General). But Diplock did manage to undertake the odd private instruction. In Bishop v Gloucester v Cunnington [1943] KB 101, he acted for the Bishop in proceedings to repossess a house. His leader was A.T. Denning KC. They won.

Diplock returned to practice in 1945, and, in April 1948, he was appointed King's Counsel. He was the most junior appointee that year. (Immediately above him was Charles Ritchie Russell, grandson of early Commercial Judge Lord Russell of Killowen. Russell and Diplock would sit together in the Court of Appeal in the 1960's, and in the House of Lords in the 1970's.) Allowing for the fact that his career had been largely suspended during the War, he had been in practice for little more than a decade. The pace of his advance was striking, and it hardly seems credible to suggest that it was justified by outstanding success at the Bar. Diplock appeared in no cases in Lloyd's between 1945 and 1948, and in only a handful in the official reports (a rag-bag of employment, local government, and, in particular, agricultural land and rent control). His link to the Master of the Rolls may have hastened his rise. But it also seems plausible that he was being rewarded for wartime services which could not be more openly acknowledged.

Within weeks of his appointment to the Bench, Diplock was trying a case about a cargo ship. But it was one of the interminable procession of personal injuries claims arising from accidents on docksides or ships in harbour which filled Lloyd's Reports in the 1950's. The editors had little other shipping-related litigation to report. Diplock's appointment co-incided with the years when the Commercial Court was so quiet that a single Judge could dispose of its business in a few days each month. With Patrick Devlin, William McNair, and Colin Pearson already available to deal with such negligible work as there was, the Court had no immediate need to call on Diplock. He did not hear any reported commercial cases until 1958, and only about forty in all, the result of lack of opportunity, not want of ability. As first instance Judge in Scruttons v Midland [1959] 2 QB 171, he held that a sub-contractor was not automatically entitled to the benefit of exemptions and limitations in the head contract, in disregard of the contrary suggestions by J.A. Hamilton and T.E. Scrutton in The 'Grelwen' [1924] AC 522. The Court of Appeal and House of Lords upheld him. They did so again in Tsakiroglou v Noblee Thorl [1958] 2 Lloyd's Rep 515, in which Diplock held that a CIF contract for the sale of East African cargo from was not frustrated by the closure of the Suez canal. His most eye-catching decision was Port Line v Ben Line [1958] 2 QB 146, in which time charterers claimed that purchasers of the ship was bound by the charter. They had in their favour the decision of a low-voltage Privy Council in Lord Strathcona v Dominion Coal [1926] AC 108. But Strathcona did not fit with the principle that a time charterer acquires no interest in the ship. Although Privy Council decisions are not technically binding in England, a less self-assured (or intellectually rigorous) Judge might have followed Strathcona and left the Court of Appeal to sort things out. But Diplock had the courage to stand by his conviction that Strathcona was wrong.

The ‘Benvannoch’ (formerly ‘Silveroak’), the subject of the dispute in Port Line v Ben Line. It was scrapped in 1969.

Diplock as a Lord Justice of Appeal in the early 1960’s (top), and on his introduction to the House of Lords in 1968.

In seventeen years as a Law Lord, Diplock sat on more than five hundred House of Lords and Privy Council appeals in almost every area of law. It was in the Lords that his interest in judicial review, peculiar in someone so clever, but possibly triggered by his Privy Council practice, became prominent. But his decisions also included a fair proportion of altogether more intersting and intellectualy challenging commercial cases. In The 'Albazero' [1977] AC 774, he explored and explained the law on the recovery of damages in contract for losses suffered by third parties. The 'Miramar'[1984] AC 676 remains the leading authority on the incorporation of charter terms into bills of lading. In The 'Scaptrade' [1983] 2 AC 694, Diplock revisited Port Line v Ben Line territory to hold that a time charterer, having no interest in the ship, cannot invoke the principle of relief against forfeiture. And in a series of landmark cases culminating in Photo Productions v Securicor [1980] AC 827, he developed his thinking begun in The 'Hong Kong Fir' to analyse the effects of repudiatory or ("fundamental") breaches of contract and demolish Lord Denning's theory that repudiation automatically deprived the defendant of the benefit of exlcusion or limitation clauses. Taken with The 'Hong Kong Fir', these decisions had a subtle but nonetheless transformative effect on contract law. They may Judges increasingly reluctant to categorise contract clauses as conditions, and shifted the focus of termination for breach towards the concept repudiation. Diplock also contributed to the move from literal to contextual interpretation of contracts, with a celebrated dictum about the importance of intepreting commercial contracts in accordance with commercial common sense (although it is doubtful whether there ever was a time when Commercial Judges adopted a strictly literal approach).

In addition to his judicial work, Diplock spent a decade chairing the Security Commission, which investigated serious state security breaches, a role for which his time on the Security Executive must have made him a natural choice. He was appointed to investigate ways of preventing jury-intimidation or partiality in Northern Ireland terrorist trials. The non-jury "Diplock Courts" which were his recommendation made his name well-known to non-lawyers. As Law Lord, he became an enormously significant figure in public life.

Diplock's judgment also became less reliable. R v Caldwell [1982] AC 341, in which he decided that (in effect) moral wrongdoing was not a necessary ingredient of crimes of recklessness, was received with near-universal condemnation. It was overruled twenty years later, on the sound grounds that it was "apt to cause injustice and offended a jury's sense of fairness" (R v G [2004] 1 AC 1034). O'Reilly v Mackman 1983] 2 AC 237, in which Diplock tried to channel all challenges to state entities into judicial review, with its strict time-limits and other constraints, caused similar unease. It was heavily qualified by subsequent decisions. In the commercial field, Diplock's suggestion in The 'Afovos' [1983] 2 AC 94 that there can be no anticipatory breach of a condition has generally been disregarded. In truth, Diplock went on past his prime. If he had left office at the end of the 1970's, he might well have retained the deservedly high reputation of his heyday. But he was the last serving Judge who had been appointed before the introduction of the compulsory retirement age, and could not be got rid of.

Like Robert Wright and Adair Roche, Diplock was a devoted horseman. He rode in the Bar and Bench annual point-to-point event until he was seventy. (Apparently he also liked football, but presumably did not play the game into his seventh decade.) Diplock once said that a morning gallop before Court "puts me in a good humour", so perhaps having to give up riding late in life contributed to the roughening of his temper during his later years. He called one favourite horse "Circuit" (or, in one version of the story, "Assize" - unless they were separate beasts) so that, should anyone call while he was out hacking across the countryside, his clerk could answer truthfully that he was unavailable because he was out on Circuit. (No-one ever claimed that Diplock’s sense of humour was as well-developed as his intelligence.) A daring rider, he collected a few knocks over the years. In 1958, he broke his arm in a weekend fall while in the middle of an Old Bailey trial, and came into Court on the Monday with his arm in a sling beneath his red robes.

“There are, however, many contractual undertakings of a more complex character which cannot be categorized as being “conditions” or “warranties”. Of such undertakings all that can be predicated is that some breaches will and others will not give rise to an event which will deprive the party not in default of substantially the whole benefit which it was intended that he should obtain from the contract.”
The ‘Hong Kong Fir’ (1961)

"A contract is the source of primary legal obligations upon each party to it to procure that whatever he has promised will be done, is done… Breaches of primary obligations give rise to substituted or secondary obligations… The contract is just as much the source of secondary obligations as it is of primary obligations.”
Photo Production v Securicor (1980)

“If detailed semantic and syntactical analysis of words in a commercial contract is going to lead to a conclusion that flouts business commonsense, it must be made to yield to business commonsense.”
The ‘Antaios’ (1983)

Diplock’s reflections on the law of contract were profoundly influential, although some were perhaps more illuminating than others.