But recently declassified MI5 files shed more light on Hess’s mysterious flight to Scotland, and finally prove the conspiracy theories to be unfounded, according to the duke’s son, Lord James Douglas-Hamilton, the Lothians MSP.
The Conservative peer said yesterday that the new MI5 files relate to a letter intended for his father, the 14th Duke of Hamilton, then commander of RAF Turnhouse in Edinburgh, which was intercepted by the British secret services.
The communiqu, which was sent by Hess’s personal adviser, Albrecht Haushofer, in September 1940, had suggested a meeting with the duke in Lisbon in a secret attempt to achieve a peace settlement.
Five months elapsed before the duke was contacted by RAF intelligence and informed of the letter, as the original went missing and MI5 ran security checks on it. It was then decided that any meeting with the duke would not provide sufficient opportunity to double-cross the Nazis, and therefore the request went unanswered.
At the time, neither the duke nor MI5 knew that Hess was behind the letter.
Three months later, on 10 May 1941, Hess became increasingly desperate to explain matters personally to the British and embarked on his ultimately failed mission in the hope of exploiting the duke’s connections with the government and the monarchy to secure a peace deal with Germany. Amid competing theories about who knew what of the episode, the new papers offer a fresh insight into the work of British intelligence in the eight months between the letter being sent and Hess parachuting into a field in South Lanarkshire.
Lord James, whose writings on the subject include The Truth About Rudolf Hess, published in 1993, said the documents proved that his father had no idea Hitler’s deputy had planned to embark on his unauthorised mission to broker a peace deal.
He said: "This letter was never answered, and these files show that MI5 played an unwitting role in Hess’s flight to Britain, since any answer would have made his flight to Britain unnecessary.
"For 35 years I have been wanting to get my hands on these files, now slipped almost surreptitiously into the public domain, and they corroborate and confirm the accuracy of the account in The Truth About Rudolf Hess.
"From the papers, it is absolutely clear that neither MI5 nor my father had any inkling Hess was involved, nor did either have any interest in peace negotiation with the Third Reich."
Lord James added the papers cleared the reputations of both MI5 and his father.
He said: "This kills off conspiracy theories that there was a traitorous plot by the British intelligence services to broker a peace deal, and it is a shame it has taken so long for this information to come out, as it proves that no dishonour can be attached to MI5 or my father from the Hess episode.
"Although Hess had never met my father, he admired his skill as a pioneering aviator and thought his connections would help him to broker peace for Germany. My father did know Albrecht Haushofer who wrote to my father on behalf of Hess, but when the pair met after his crash it was a great shock to my father."
Professor David Stafford, project director for the Centre for Second World War Studies at Edinburgh University and expert on British Intelligence, agreed the files prove MI5 was not involved with Hess.
"If we boil it down very simply, the conspiracy theory is that there was a plot by British intelligence deliberately to lure Rudolf Hess to Britain to engage in peace negotiations in the spring of 1941," he said.
"But I think these documents knock that theory very firmly on the head. Hess deluded himself into thinking that he could broker a peace deal and thought he could secure the peace that Chamberlain thought he had gained when he flew to Munich before the war started."
In fact, British intelligence did consider using the duke as a double agent, and Colonel Tar Robertson, the head of MI5’s double agent section, wrote: "He (Hamilton) is a slow-witted man, but at the same time he gets there in the end; and I feel that if he is properly schooled before leaving for Lisbon he could do a very useful job of work."
However, it was decided it was too late to take advantage of the situation for "disinformation and double cross".
Lord James suggested that Hess’s involvement in any plan would have been dangerous for Britain and "thanked God" his father was not sent to Lisbon after all.
"Even the quick-witted Tar Robertson was not sufficiently quick-witted to realise Rudolf Hess was involved," he added.
Official records, disputed by some, state that Hess flew to Scotland from Germany in May 1941 in an attempt to discuss a peace settlement with the duke and the British establishment. He crash-landed 12 miles short of the duke’s estate at Dungavel, near Glasgow, and was arrested. His peace proposals were rejected.
He was eventually tried for war crimes at Nuremberg and sentenced to life imprisonment at Spandau prison, where he committed suicide in 1987.
But for years conspiracy theorists, including Hess’s own son, Wolf Rudiger Hess, have suggested British intelligence lured Hess to Britain with a proffered peace deal between Britain and Germany.
Lord James said that the documents proving neither his father nor MI5 was involved in negotiations with Hess came "better late than never", but he criticised the government for withholding them for so long.
He said: "By maintaining excessive secrecy in relation to Second World War matters, those ministers responsible for that secrecy were actually doing Britain a disservice, because Britain had not broken its honourable record and they should have been more transparent in showing that to the British people.
"My father initially believed that if Hitler and the Nazis could be removed then it would be worthwhile to negotiate with Germany in the interests of saving lives. It later became clear to him that Churchill was right - the way to deal with Hitler was to fight until the bitter end."