High on a wall in Westminster Abbey, in anything but pride of place, is Richard Westmacott's monument to Spencer Perceval, who, 200 years ago today, was shot dead in the House of Commons, the only British prime minister so far to have died by assassination. The memorial was "erected by the Prince Regent and parliament", and perhaps it would have been given a more prominent position if the prince had not been a late and reluctant supporter of Perceval's ministry. His straitlaced evangelical premier had been especially unhelpful in the matter of Princess Caroline, the notorious supposed adulteress. Reasonably enough, Perceval regarded her as much more sinned against than sinning and, by some skilful moral blackmail, had forced the prince to receive her again as his wife.

But then again, had the monument been more visible, its oddly ambiguous message would have been more awkwardly legible. It consists in part of a relief depicting the scene a moment after the shooting. Perceval, dead or dying, is supported by two of his political allies in the campaign for the abolition of the slave trade. Gathered round are various other politicians, shocked and sorrowful; but it is the assassin, John Bellingham, who commands our attention. Taller than everyone else, he has the bearing of a man conscious of having performed an act of shining virtue, a good deed in a naughty world. He is under arrest, but clearly has no intention of escaping, is quite willing to take responsibility for what he has done.

It is hard to escape the conclusion that Westmacott, or the prince, or Perceval's successor, Lord Liverpool, was suggesting that between the pious Perceval and his cold-blooded assassin there was something to be said on both sides. The public was less fair-minded. Britain was in deep recession, for which Perceval was blamed; the news of his murder was greeted with jubilation all over London, and soon all over Britain. Bellingham, for the week of life he had left, became a celebrity, almost a hero.

In front of the relief lies a larger sculpture of Perceval dead, and at his head sits the allegorical figure of Power, mourning her protégé, who had exercised his office with the utter single-mindedness of one who knows that there is no alternative to his policies. "He was a bitter persecutor," remarked his political opponent Lord Holland, "of such political and religious principles as he, without much painful inquiry or dispassionate reflection, disapproved." At Perceval's feet stand two more allegorical figures, weeping for his death. One of them is Truth, "nuda veritas", stripped to the waist to show that she has nothing to hide, but by a judicious positioning of her left arm managing to hide that nothing from us. Andro Linklater believes that the full truth about the assassination has always been hidden, and has written this book to explain why Perceval had to die. In the process he offers a fascinating account of Perceval's enigmatically simple character and crafty politics.

Bellingham insisted there was no mystery about what he had done, no secret accomplice, no motive other than the one he was only too willing to declare, to everyone, at length: even on the scaffold, standing on the trapdoor with the rope actually round his neck, he started explaining it to the chaplain of Newgate. He was a merchant from Liverpool who had become involved in the trade with Russia, and at the port of Archangel, now known as Arkhangelsk, had been imprisoned for a fraud he did not commit. He lost thereby a sum amounting to many hundreds of thousands of pounds in today's money. He had appealed for help to the British ambassador in St Petersburg, who passed the case on to the consul, who did little to help. Thus, when eventually released and back in Britain, Bellingham regarded the government as morally bound to indemnify him for his losses; the future of his wife and children depended on the recovery of the amount he had lost.

It was the right of every man, Bellingham believed, to petition parliament for the redress of grievances, but Perceval insisted that the government had no obligation to recompense him, and refused to receive his petition. Obviously enough, or so it seemed to Bellingham, his only remaining chance of a remedy was to kill the prime minister. He had no personal grudge against Perceval; to kill him would be a simple act of justice; and when at his trial he explained the reasons for his action, he would of course be acquitted and indemnified. His counsel pleaded that he was insane, but Bellingham would have none of it: in his position, anyone would have done what he did.

The law officers, determined to hang Bellingham in short order, put him on trial a week after the killing. They did not want to waste time looking for accomplices, and agreed with Bellingham that he was quite sane. They produced evidence to show how, weeks before, he had bought pistols, had a secret pocket made in his coat to conceal one of them, and had sat in the public gallery of the Commons, studying Perceval so there would be no chance of killing the wrong man. Surely these were the actions of a sane man acting with malice aforethought? Not so, replied Bellingham: he had certainly acted with forethought, like an executioner, but not with malice. He was no murderer.

Linklater does not doubt that Bellingham was sincere in insisting that he had acted alone, and for the reasons he gave. But Bellingham, he suggests, was also the unknowing instrument of more powerful forces, with vastly more to gain than Bellingham by the death of Perceval. The argument that sustains this claim is ingenious and almost convincing.

Perceval was shot on the day the Commons was debating a motion by Henry Brougham to rescind the notorious "orders in council", the chief plank in Perceval's policy for the defeat of Napoleon. In 1806 Napoleon had attempted to impose the "continental system", which forbade the allies of France, and the nations conquered by Napoleon, to trade with Britain and Ireland. The orders in council were Perceval's retaliation: as well as banning trade with France and its allies, they forbade neutral nations to trade with France, and gave the British navy the pretended right to board all neutral ships in search of goods destined for France. Napoleon responded with decrees against neutral ships sailing to UK ports, and the US enacted an embargo on trade with all the belligerent nations.

By the end of 1811, a deep recession and credit crunch had settled over Europe and North America. The value of British exports and imports had fallen by 75%. Liverpool was especially hard-hit, for it had lost much by the collapse of the slave trade and was now hugely dependent on trade with the US, half of which passed through its port. That trade too had now collapsed, and with it the increasingly valuable stream between Russia and the States, which also went through Liverpool from Archangel, the only Russian port France could not blockade. In Washington there were calls for war with Britain, which came, after Perceval's death, in 1812. In Britain there were demands from merchants, shippers, manufacturers and workers for the orders in council to be rescinded. Few doubted that Perceval would resist, and that the orders would stay in place until Napoleon was defeated or Perceval ceased to be prime minister  both apparently distant prospects. When Brougham proposed his motion, Perceval stayed away, but was noisily summoned to the Commons to defend his policy, and was on his way to the chamber when Bellingham shot him at point-blank range. A month later Lord Liverpool, of all appropriate titles, became prime minister, the orders in council "evaporated", and the economy began to recover.

Bellingham had been in London since January, attempting to present his petitions and then preparing the assassination. By the end of that month he was flat broke, but from February, his accounts suggest, he was reasonably flush. Linklater believes that he was being funded by two men, closely associated: Thomas Wilson, a London merchant and banker to the trade with Russia, and Elisha Peck, an American businessman resident in Liverpool, men with fortunes to lose if the orders in council continued in force, and with every reason to wish Perceval dead. One or both may have been employing Bellingham, in a small way, as their agent; both would probably have heard him declare that if Perceval did not make him proper restitution, he would kill the premier. Both had every reason to fund Bellingham until he was driven to make his attempt, without Bellingham ever understanding how they were using him. Linklater's evidence for this account is intriguing, though here and there it has to depend on conjectures that, in the space of a page or so, are wished into hard facts. His case is impossible to prove, but too plausible and too much fun to ignore.

 John Barrell's The Spirit of Despotism: Invasions of Privacy in the 1790s is published by OUP.