Harry had had enough of the grand sweep of history. 'Well, all I want is to get back to my own life,' he said firmly. 'I'm thirty-six years old now, brother, and feeling it. I have spent too long on the past; I want to think of the future. If I can find a wife, have some children-'

'Good for you.'

'And I want to get back to trade. If there's profit to be made from this adventure, I want a share in generating it – and spending it. The Spanish aren't the only explorers. I have been corresponding with an Italian adventurer called Jacobo Caboto who is going to approach King Henry with a proposal to try to find a route to Asia through the northern seas, following in the wake of the long-dead Vikings.'

Geoffrey grunted. 'Perhaps you will find Vinland, if not China.'

'And what next for you, Geoffrey?'

Geoffrey thought, tugging at his lip. 'I have been contemplating this muddy business we have been involved in. I want to do something about it. We have been at the focus of a battle between two time-meddling agents, the Weaver who fed the designs of the Engines of God back to Aethelred and Aethelmaer, and the Witness who spoke to Eadgyth.

'And there is evidence of other tampering, Harry. Your remote ancestor Sihtric, dead four centuries, made marginal notes on his copy of the Engine Codex. He believed he had found evidence that the Weaver, or another agent, tried to avert the course of the Norman Conquest of England. And even earlier, in the deep and ancient days when the Romans ruled Britain, another meddler, or the same one, tried to arrange the assassination of the Emperor Constantine. I have tried to map all this.'

He produced a parchment on which he had scribbled a kind of tapestry. He explained that the long warp threads were the true course of history, the wefts the deflections of history, or attempted deflections – he had found no less than six of them, from the failed assassination of Constantine to the amulet of Bohemond which resulted in the murder of the Mongol Khan.

Harry gaped. 'That's astonishing.'

'Yet it seems to be true. But after all this meddling, what is left? What is true, what is right? What should our destiny be? The tapestry of time, rent apart and rewoven over and over, has become a shabby, worthless rag.'

'And you're thinking of doing something about it?'

Geoffrey's gaze was distant. 'What if I could speak to a Weaver of my own, off in some future time? I don't know how – perhaps I will write and speak and have my words printed up in a thousand copies, so that they never die. Or perhaps I will simply take my testament to my grave, and one day, when my body is exhumed, my words will rise with me.'

'What will you say?'

'I will ask for help. I will ask that these meddlers, the Weavers and the Witnesses and the whole pack of them, be halted from their tampering with time's tapestry. That an end be put to it, and history be allowed to resume its proper course.'

Harry looked around the world, at the huge blue sky of Spain, the shoulders of the land around the harbour, the endless shimmering sea. He felt the heat of the sun on his neck, smelled the ocean salt, heard the cry of seabirds who wheeled far above. It was all so vivid, so specific, so real, that it seemed impossible to him that it could all be changed on a whim.

But of all the elements in the pretty panorama before him, the most vivid and exciting were the brave forms of Colon's three ships.

At last the ships cast off, to distant cheers from the crowds who thronged the harbour. They put out to sea towed by launches, but soon their sails billowed with the offshore wind, and they surged into the waters of the Ocean Sea, the cross of Christ bright red on their sails.

'He has gone,' Harry said.

'Yes. In this moment it changes,' Geoffrey murmured. 'If Colon is right, perhaps this is the moment when Christendom will win a world, and will never again be threatened, its destiny never again subject to the whim of history, a battle turned, a ruler dying. Certainly the dark future warned of by Eadgyth's Witness has vanished, like so many others: whole histories that never existed, and never will exist. Millions of lives, generations of men and women living and loving, fighting and dying, marching off into the future like legions – all of them wiped out!'

Harry said, 'But after all this, Geoffrey, as Colon sails, has the world changed for the better, or worse?'

'I doubt if even the Lord God would answer that with confidence, my friend.'

They watched until the three fragile ships, pressing ever west, disappeared over the curve of the round world. Harry felt as if a nightmare that had plagued him since his father's death was at last sailing out of his head.

Then, thirsty for wine, they made their way back down the slope and into the town.

The elderly parish priest, the same Arthur who had cherished her before, walked with Agnes to the church.

It was a hot, humid day, and England was a feverish green, lacking colour now the flowers of early summer had gone, and the air was full of busy insects. It was quite a contrast to the brilliant, arid severity of Spain. Yet Agnes was glad to be back, for this was her home. Even the ache in her damaged mouth didn't seem so bad here.

They came to her old cell, still fixed on the wall of the church. It was strange to Agnes to peer on it from outside. It looked so tiny, yet once it had been her whole world.

But it had changed. The squint, the one tiny window, had been stopped up with loose brick.

Arthur said regretfully, 'I'm afraid we have to contain her. The noises she makes are sometimes rather disturbing. The screams, you know. The yelling in half a dozen languages. Some of my flock believe she is possessed, not by the Holy Spirit but by His malevolent counterpart. Oh, we miss you, Agnes! There's no other word for it. My flock were drawn to your piety and your patience. They loved you in a way they will never love her.'

Somewhere in Agnes's bruised heart, she was touched. She had a bit of parchment with her and some charcoal. She scribbled, But she more interesting. True word of God in her rambling. Truth that underlies the universe. Chaos of it all.

He looked at her strangely. 'You have changed. You have been hurt.'

Hurt long before I went into cell. Won't go back. Other plans.

He nodded, accepting, regretful, wary.

On impulse Agnes bent and pulled one brick out of the stopped-up squint.

There was a rat-like scrabbling, and a single pale eye was pressed to the hole, its pupil huge from the darkness within. 'You. You hell-spawned witch! I should have destroyed you while I had the chance-'

Agnes rammed the brick back in the hole.

'You see what I mean about stopping her up,' said the priest, troubled.

Better this than Inquisition. By the time she's got her grave dug out she'll find peace.

'I pray you are right,' said the priest.

Agnes and Arthur walked away from the church, while the verdant life of the English summer rustled and swarmed around them. But if she listened hard, Agnes could still make out the cries of the woman trapped in the brick cell.

'I am Grace Bigod. My ancestors fought with the Conqueror, took the Cross, and built a holy kingdom in the Outremer. I do not deserve this – not this! Let me out, oh, let me out…'

Afterword

I'm very grateful to Adam Roberts for his expert assistance with the Latin of the Incendium Dei cryptogram.

A general history of Islamic Spain is Richard Fletcher's Moorish Spain (Weidenfield amp; Nicolson, 1992). I have used Fletcher, and the Encyclopaedia Britannica (2001 edition), as references for variant spellings of personal names, both Moorish and Christianised.

I also used Fletcher as a reference for spellings of Spanish place names, which varied through the period covered here, and many sites had both Christian and Moorish names. I have used the term 'al-Andalus' to mean that part of the Iberian peninsula under the control of the Moors at any given time, and 'Spain' to mean the Iberian peninsula itself; the peninsula of course includes modern Spain and Portugal, both of which coalesced as political entities during the period up to 1492. Regarding names in England I have generally defaulted to modern versions. In all my choices I have aimed primarily for clarity.