‘By the year of Our Lord 793,’ Sihtric said, ‘in which your Viking ancestors first raided England, Orm, there were Moors in Paris and in Rome. Even Constantinople had fallen, after a decade-long siege from both east and west. After that the political history of Moorish Europe was no simpler or less fractious than that of al-Andalus, but overall the Moorish grip on the Great Land never loosened.

‘There was to be no Jorvik, no Danelaw, and no Normandy. There was no battle at Hastings, no Norman invasion of England - for there were no Normans! The emirs never allowed Vikings to settle on their territory as the Frankish kings did.’

Armed with the legacy of antiquity, the Moors were able to make the northern lands flourish as they had al-Andalus. Populations rose steadily, and gained in wealth and health - and, just as steadily, converted to Islam. There was an intellectual revolution, and marvellous medicines and machines transformed the lives of the people.

‘The greatest mosque in Europe was built in Seville, but the second grandest was in Paris,’ Sihtric said. ‘The greatest library in the world was in London. Think of that!

‘And it was in a Moorish London that a young man called al-Hafredi was to be born. In a few words he sketches his London for us, a London where minarets and marble-columned palaces rise within the old Roman walls, and the cries of the imams drift across the Thames.

‘Al-Hafredi claimed he had come from a far future, a thousand years beyond the Muslim conquest. And he sketches that millennium - a future that was already history to him. There will be invaders,’ Sihtric said. ‘From the east. A wave of savage horsemen, bursting out of Asia. The Muslim rulers, fractious as ever, will be unable to stand before them. Al-Hafredi details their progress. But the nomads’ world empire will be brief, gone in a few generations, leaving only memories of distant lands.

‘In the next age, plague. Many will fall. It would have been far worse, says al-Hafredi, if not for Muslim medicine.

‘And in the age after that there will be a terrible war, a war of the Silk Road, as empires of east and west fall on each other. The war will engulf the whole world, and will last another century. And it will be won by machines. I imagine engines like mine, like Aethelmaer’s, or even more destructive, born in the fecund minds of warriors and those who serve them.

‘The war, long and bloody, will be won by the Muslims. In the end Islam will hold sway across all the world as it is known, from Scandinavia to Africa, from Ireland to India and the lands beyond. And ships bearing the crescent banner will sail far beyond the horizon in search of new lands to conquer, new peoples to convert.’

‘And somewhere in this future Islamic world,’ Orm said, ‘your friend al-Hafredi broods, unforgiving.’