'Now that,' she said, 'is a delicious thought.'

They arrived at last at a patio with an elaborate sunken garden surrounded by graceful arches. Peter heard laughter, and in the shadowed spaces beyond he saw lithe running figures. At the western side of the patio they were brought through an elaborate horseshoe-arched doorway into a hall called the turayya. This grand hall was the emir's throne room; beneath a domed roof pillars of fine marble supported complex arcades. The chamberlain said it was named for the star system called in the west the Pleiades. It was so called because the rooms around this central hall were set out like those stars in the sky.

Peter's eye was drawn to the tile work which adorned the lower walls. A pattern of black stars, rich blue rectangles and white strips, repeating endlessly, somehow covered every scrap of the surface. 'I think I could spend my whole life,' Peter said, 'just looking at the tiles on that wall.'

'I'm afraid I have other plans for you,' Subh said. 'Enough gawping. Come now and sit with me.'

He joined her on floor cushions, and allowed a servant to pour him a glass of the juice of crushed oranges, as they waited for the first of their scholarly guests to come and talk of engines of war.

It was two years before Joan and Saladin saw Thomas again. They were years in which troubles more urgent than the vague promises of prophecies rose up and overwhelmed them – and, at last, after generations, the family of Robert the Wolf was forced to flee from Jerusalem.

Saladin thought of England as home, for this was the birthplace of his ancestor Robert; this was where his family had its deepest roots. But to a boy from the Outremer it was a strange, dark place. The sun never seemed to climb high in the sky, and was somehow dim even at noon. And Saladin felt cold, cold to his bones.

But as their ship sailed cautiously up a great estuary and into London he clung to the rail, staring out curiously at a city that sprawled across the horizon. Even the river was crowded, busy with trade, and upstream of a grim fortress called the Tower wooden cranes like long-necked birds pecked at ships laden with English wool, or with silk and wine coming into the country from the continent.

Thomas Busshe met them off the ship. Saladin was glad to see a familiar face in this strange country. Thomas had arranged lodgings for them for the night at a Franciscan priory. He had aged in the two years since their last meeting, and walked with a limp. But he seemed excited and pleased to see them – indeed, bursting with news, as he led them into the city.

As they walked, it was the filth that struck Saladin. All the narrow streets just ran with human sewage. And butchers worked out in the street, making the cobbles a mess of offal and bone fought over by rats and crows and bloody-handed urchins. Though squads of beadles, under-beadles and rakers swept gutters and drains clear of dung and hauled animal remains to the rivers for dumping, the stench of ordure and blood was overwhelming.

But, noisy, crowded, jostling, swaggering, stinking London bustled with commerce. In Jerusalem, tension simmered and arms and armour were everywhere. This sprawling city seemed to be run by merchants and shopkeepers, not by soldiers.

And there were no Saracens here, so Saladin's dark colouring stood out. A gang of well-dressed young men spotted Saladin and thought him a Saracen himself. 'Do you eat babies? Do you screw your mother? Get back where you came from, carpet-biter!' Thomas Busshe restrained him before he could draw his sword, but Saladin seethed.

That night he tried to sleep in air so dense and smoky he could barely breathe. He longed for the hot light of the Outremer, the iron scent of the desert.

In the morning, in Thomas's priory, they spoke of the fall of Jerusalem.

Joan bit into the tough bread softened by a bit of broth that passed for food here. 'Well, in the end we had to flee. We all did. Emperor Frederick's truce expired even before your visit two years ago, Thomas. But the Muslims were squabbling among themselves: brothers, rival princes in Damascus and Egypt, waging war on each other. That saved us for a bit. But at last, this year, one victorious princeling allied himself with the Khorezmians of Syria and took back Syria and Palestine, and – well. Reunited, the Muslims regained the Holy City too.

'We had warning. We fled with what we could carry, which was little enough. In Acre we bought our way onto one of the last ships – hideously overcrowded, you can imagine. The cost was obscene.'

Saladin was impatient. 'You talk of money, mother. What does money matter? This was not Saladin, not honourable. These Saracens burned the Church of the Holy Sepulchre. They massacred our people!'

Joan nodded, weary, grim, looking older than her thirty-two years. Saladin thought her experiences had hardened her, that she had come to hate the Saracens as perhaps she had not before. 'My son would have stayed to fight. I forced him to flee with me, for I needed his protection. Any shame in our flight is mine, not his.'

'Yes.' Thomas looked at Saladin. 'And this is what you must tell your confessor.'

Joan said, 'So Robert's blood descendants return to England half destitute. I may yet need the charity of your house, good brother.'

Thomas touched her hand. 'It won't come to that. Put away your despair; cling to hope. Your fortunes will rise again… I have news for you.'

Joan studied him. 'You're being enigmatic, brother. Spit it out, man.'

'Not here.' He glanced around the empty room. 'London is a nest of spies.'

'Even here, in this priory?'

'Monks must make a living, lady. We must operate in the world of money and power. It's a corrupting environment. I will bring you to my own house, which is far from London. There we can talk with confidence.'

Saladin groaned. 'More travelling?'

'No more ships,' Thomas said. 'I promise you that.'

'But what is it you want to tell us?'Joan demanded. 'Give us a hint at least.'

Thomas smiled. 'Very well. I have come to believe more firmly than ever that your family legend of war engines and prophecies may have some truth in it. For I have stumbled upon evidence that such things have happened more than once. Warnings from the future, leaked into the past. No, not just evidence – I believe, proof. Eyewitness proof.'

Joan and Saladin stared at him. But he would say no more.

The next day they journeyed north out of London, jolting over an old Roman road, not repaired for centuries. Saladin longed for a camel's smooth gait, but there were no camels in England.

The country was green, and even away from the city the land was crowded, Saladin thought, full of people, carpeted with farms and studded with little towns and dung-coloured villages. Waterwheels creaked by the rivers, and windmills whispered as they turned in the breeze.

They stopped for a night at a city called Colchester. Set above a river, it was surrounded by bristling walls laid out in a vast rectangle, walls founded by the Romans but built up massively by the generations since.

Thomas said the history of England was written in those battered walls. William the Conqueror's descendants had been a sorry lot, and fifty years after William's death the country was racked by a long civil war between Stephen and Matilda, two of William's grandchildren. Thus, said Thomas, the ultimate legacy of William the Bastard: a once-prosperous country ruined by fratricidal warfare, famine, extortion, carnage and chaos. Saladin had heard of Matilda's grandson Richard the Lionheart. But King Richard died not long after his final crusade. His brother John was a weak, distrustful, treacherous, deeply unpopular king who was forced to cede power to his barons in a Great Charter, and under John's son, the third King Henry, a council of nobles, called a 'parliament', began to meet in Westminster. In England, then, power was shifting. But the latest King Henry favoured the church; great cathedrals sprouted like mushrooms…