If murder was mooted in No 10, what else is true?

It was a remarkably good week for conspiracy theorists. No sooner had the latest miasma of speculation ceased to waft over the 50-year-old corpse of John F Kennedy than news sped in from Switzerland that an examination of the recently exhumed body of Yasser Arafat, the former leader of the Palestine Liberation Organisation, had revealed abnormally high levels of polonium, leading to the suspicion he had been poisoned.

Meanwhile, at Westminster the heads of the British secret services were facing questions from a parliamentary select committee of such a sensitive nature that a time delay had to be placed on the broadcast lest anything untoward should slip out. As for the revelations emanating from the Old Bailey, where the trial of the News International executives arraigned for their alleged complicity in phone hacking, these are reportable but not, alas, discussable.

Whenever I read the words "conspiracy theory" I always think of a man who, for the purposes of this article, I had better call "Ralph Davison", a former SAS officer whose autobiography, back in the 1980s, I attempted to ghost. Mr Davison, who once interrupted a telephone conversation with the disquieting intelligence that he thought his phone was being tapped, had, by his own admission, been involved in practically every high-level secret service assignment of the era. He had, he assured me, attended the Balcombe Street siege of 1975, been present at the Libyan Embassy stake-out in 1984 and participated in a stand-off with Carlos the Jackal against the unlikely backdrop of a Majorcan swimming pool. But the most chilling story of all concerned the assassination of the Egyptian president Anwar Sadat in 1981, connived at by the British government, he briskly deposed, who had withdrawn his SAS bodyguard in the certain knowledge that the now untrammelled killers would strike.

There was, and is, no way of demonstrating that this particular conspiracy theory was true, but it sounded plausible, not least because of the bluff, no-nonsense, Yorkshire-accented way in which Ralph dealt it out (the project foundered by the way, owing to his insistence that his exploits should be written up as a novel). And, it scarcely needs saying, the idea of a top-secret British scheme to destabilise the Middle East by allowing one of its major players to be taken out is exactly the kind of thing one reads about in newspapers, whether applied to plots to assassinate JFK or the very common assumption that the CIA were involved in a conspiracy to remove Harold Wilson from Number 10 in the strife-torn 1970s.

Why is the average citizen, whose knowledge of the world comes entirely from the mass media, so absorbed by these tales of quiet words in high places, stealthy backstairs intrigues and discreet – or not so discreet – wire-pulling in the ante-chambers to the corridors of power? One answer lies in the widespread belief, common to most countries with at least the vestige of a bureaucracy and a centralised administration, that much of what goes on at the upper levels of national life is arranged in secret, offering a kind of code which onlookers can amuse and edify themselves by trying to decipher. Dickens once wrote an essay on these lines entitled "It is not generally known..." ("It is not generally known that the people have nothing to do with a certain large Club which assembles in Westminster, and that the Club has nothing to do with them.")

Another rests on the habitual use made of conspiracy theories by political minorities as an authentication of the stand they are taking against the world. The right-wing pacifist groups who tried so hard to limit the UK's involvement in the Second World War, for example, believed everything that the classic conspiracy text The Protocols of the Elders of Zion told them, in particular that the pre-war Soviet politburo was largely composed of Jews and that practically all newspapers were under Zionist control.

Then, of course, there are the localised conspiracy theories that attach themselves to the idea of the "Establishment", that vast, shady and unaccountable body of civil servants, politicians, BBC mandarins and heads of Oxbridge colleges, ceaselessly at work – or so the argument goes – to frustrate the ambitions of ordinary people, ganging up to secure preferment for their own and ensure that the upper levels of society are constantly replenished by the right kind of recruits. My father's version of this secret alliance was an entity known as "them" – a gargantuan and anti-meritocratic clique designed to frustrate his and his family's progress through life. Occasional triumphs over this tribe of talent-quellers and aspiration-deniers were marked by extraordinary detonations of temperament. I can remember once shouting news of an exam result up the staircase and my father responding with a cry of "That's shown the buggers!" Against all the odds, despite the accumulation of road-blocks placed in their way by this devious agglomeration of upper-class caballers, the Taylors had won through.

Whether focusing on the unexplained death of a politician or the workings of the Establishment, conspiracy theories clearly fulfil some deep-rooted psychological need, consoling the powerless and offering the intellectual satisfaction that comes from identifying a complicated shadow-world that only you, and a few like-minded savants, can see. The really alarming thing about conspiracy theories, alas, whether focused on JFK, the polonium levels in the remains of Arafat, or the idea – quite seriously bruited about in the 1980s – that Marlboro cigarettes were manufactured by the Ku Klux Klan (proof positive, three Ks are revealed when you turn the packet inside out), is how innocuous they seem when set against the Byzantine complexities of documented reality.

To go back to the mid-70s speculation surrounding the Labour government of Harold Wilson and in particular the influence supposedly exercised on the Prime Minister by his political secretary, Marcia Williams, now Baroness Falkender, it was later revealed by Wilson's adviser Bernard Donoughue that the PM's doctor, the late Joe Stone, was so alarmed by what he believed to be Williams' fell designs that he proposed to Donoughue and his colleague Joe Haines that they should "discuss ways of taking the weight of Marcia off the Prime Minister's mind". Stone then confided that he could, if necessary, "dispose" of her in a way that would make it seem as if she had died of natural causes. "He added that he would sign the death certificate and that there would not be a problem," Donoughue recorded. In the end the plan, which Haines and Donoughue thought went a bit far, was dropped. On the other hand, when set against this real-life skulduggery in Number 10 the idea that Yasser Arafat was poisoned, or that a Marlboro packet was really only an anti-Semitic fetish, seems the merest bagatelle.