Saladin was disturbed by this bloody narrative. He had thought of England as a place of Christian peace and security – like a vast church, perhaps. But England was nothing like that. Wars had been fought out here and invasions mounted, and people were forced to huddle behind the walls of towns like fortresses. And it was all so insular. Did the posturing princes of this little country have no idea of the threat posed by the Muslims, who had taken three-quarters of Christendom – and, worse, the Mongols, who by all accounts had conquered three-quarters of the whole world?

At the very heart of Colchester was a Norman castle, a little like the brooding pile in London but even more imposing. 'The most massive keep in the whole country,' Thomas said. The castle's thick walls were heaped over a tremendous slab of concrete sunk into the sandy ground. Local legends had it that the slab had been the foundation of a great Roman temple. 'Think of the size it must have been! Who would build such a monument in this dismal place? But some of the locals claim that before the Romans came this town was already the capital of the whole of Britain.' Thomas shook his head. 'I suppose we would all like to believe we are descended from kings.'

Yet, Saladin thought, that mighty concrete slab had been poured into the ground by somebody, for some purpose. But his brief flicker of historical curiosity quickly died.

Thomas led them to his priory, a few miles outside the walls of the city. It was a modest house of a few dozen monks, supported by the sale of wool and tithes paid by the inhabitants of a small village, through which they had to walk. The houses were long, leading back from a central trampled track. It seemed that a family lived in each house along with their animals: there were no barns or sheep-pens or pig-sties, only the houses, for people and animals to share. The smoky air stank of the dung used to fuel the fires. A litter of grimy children followed the travellers, wide-eyed.

Compared to the aridity of the Outremer, the land here in the heart of England was green, and so wet that wherever you saw a ditch you had to assume it was for drainage, not irrigation. But the villagers scraping away at their long, skinny fields looked half-starved and exhausted. And there seemed to be an awful lot of children here, a lot of mouths to feed; no wonder the villagers had to work so hard.

The next morning was a Sunday, and Saladin and his mother worshipped in the village's small parish church. The church was dark and cramped, but its walls were brightly painted, covered with pictures based on Bible stories and the lives of the saints. Most striking was a very severe Christ, whose image stood above the chancel arch. The righteous climbed a ladder towards Him on one side of the arch, and the damned fell screaming into perdition on the other. The villagers, listening to their priest's mysterious Latin words, smelled of their fields, of grass and earth and dung.

After the service, Thomas said he would speak to them at last of his discoveries.

In a cramped, smoky visitors' room in the priory, Thomas served them mead. Saladin sipped from his cup. The flavour was disgusting, the drink a kind of fermented honey, but it delivered a strong kick.

'Listen to the story I have to tell you,' Thomas said. 'Listen, believe, try to understand.'

He had come upon this truth by accident, when Thomas, in the service of Joan, was studying the progress of the Mongols.

Armed with al-Hafredi's glimpse of the future, Joan and her ancestors had been able to profit from a foreknowledge of the Mongols' advance. But in the year 1242 the Mongols had suddenly withdrawn from Europe. Thomas, digging into the reasons for this reversal, eventually found a man who had actually been at the court of the Mongols in that crucial season, two years ago. He was a knight called Philip of Marseilles. Devout, strong, fearless, Philip had taken the Cross more than once.

And he had agreed to serve as a legate for the Pope, in the pontiff's hopeful negotiations with the Mongol Great Khan.

The Mongols had been a nomadic people, one of many who hunted and warred across the vast grassy ocean of the Asian steppes. The Mongols' expansion across the world was the dream of Temujin, who called himself Genghis Khan, which meant 'the emperor of mankind', and he taught the Mongols to believe that they and only they were born to rule the whole world.

Genghis first unleashed his war dogs against China to the east. With one ancient civilisation reduced, the Mongols next assaulted the prosperous Islamic states to their west and south, especially Khwarazm, where they shattered an irrigation system that had endured since antiquity. Then Genghis's son Ogodai assaulted the Viking-founded cities of Russia to the north. Mongols cared nothing for cities or civilisation; resolute nomads, they wanted only plunder, and space to graze their horses.

Then the Mongols turned west, to Europe, and Christendom.

The great general Sabotai led the attack. He split his forces into three, making diversionary thrusts to north and south while Sabotai himself led the main body of his forces across the Hungarian plain. Thus Sabotai controlled forces separated by hundreds of miles and by mountain ranges; there had been nothing like this coordination and control since the legions. And the forces of the Hungarian King Bela broke before these savage horsemen, their leathery dress strange, their horses small, fast and muscular.

Sabotai set up his yurts on the plain of northern Europe, and, in the autumn of 1241, prepared to overwinter before his next push west.

He was only a few days' ride from Vienna, along the Danube. No Christian army had even slowed the Mongol advance, let alone halted it. Now it was a dagger held over the heart of Europe, a world empire preparing to overwhelm the petty, squabbling states of Christendom.

And yet Pope Innocent IV tried to deal with the Mongols. Even as the horses of Sabotai grazed east of Vienna, Philip of Marseilles was attached to a party of clerics and knights despatched to the court of Ogodai, son of Genghis Khan.

Many Christians had applauded as the Mongol attacks thrust into the soft belly of Islam. There were even hopeful rumours that some of the Mongols were Christians, adherents of a heretical sect called Nestorians who clung to an obscure argument about the separation of the divine and human nature of Christ. And there was a popular legend of a figure called Prester John, said to be descended from one of the magi who attended the birth of Christ, a Christian ruler of a vast kingdom in the east. So in the Pope's counsels there was hope that the Mongols could be turned into allies against Islam, the ultimate foe.

Thus Philip and his party came to the strange capital of the khans, deep in their Asian homeland. This was a 'city' of nomads, a town of tents, each laden with pointless heaps of booty. And yet in this place they found embassies from across the known world and beyond. The Mongols' destruction was terrible, but the unity they had imposed connected empires which had had little knowledge of each other since antiquity.

But Philip found that Ogodai, a clever, impulsive, hard-drinking man, was no Christian, no Prester John. In fact only a few Mongols were Nestorians; the rest adhered to a kind of primitive animism. And besides the Mongols waged war not for religion but for the conquest itself. To Ogodai even the Pope was no more than the weak leader of a rabble of petty states that would, in due course, be conquered, reduced and assimilated, and that was that. The Pope's embassy failed.

But it was while they remained as guests of the Khan that one of the papal party, a nervous but intelligent young monk called Bohemond, discovered in his pack an 'amulet', as he called it. He had no idea how it had got there.