In the last days / To the tail of the peacock / He will come: / The spider's spawn, the Christ-bearer / The Dove…

Long before he had ever heard of the Testament of Eadgyth, James grew up believing, or at least fearing, that the world's last days were indeed near. Legends of the last days had rattled around the house in Buxton since James had been taken in as a boy, and had listened wide-eyed to the lurid speculations of the older brothers.

As he grew, however, he learned that Franciscans had always been fascinated by legends of the Apocalypse. And as his soul and mind were opened up by the new mood of scholarship that embraced Europe, he thought he became sensible. Pragmatic. He put aside the grim prognostications, the peculiar antique longing for the end of things.

But now the quality of the whispering changed. Dreams that had once clung to the year of Our Lord 1000 accreted like ivy over another milestone year: AD 1500. That was not a remote future. That was a year James expected to live to see; he would not yet be forty.

And when the abbot took him aside one day, and showed him the abbey's secret library, where for two centuries the brothers had been labouring over spidery designs for engines of war – engines that might bring about that final catastrophe – then, in some secret library of his own soul, he began to feel afraid.

For Harry Wooler, it was the Dove himself whose beating wings cast shadows over his own life, on the day his own small world came to a kind of end, as his father lay dying.

Harry, just seventeen years old, was forced to lean over a face already like a skull, smell breath that still stank of ale, and listen while his father whispered in his ear a family tale centuries old, a tale of ancestors called Orm and Eadgyth, and a strange, dark prophecy of a man called the Dove who would shape all history. In the end this morbid tale merged seamlessly into his father's ale-drowned death-rattle. But Harry was the eldest son, and it was his turn to receive the legend, as had his father, an eldest son before him – it was his duty to listen. And after all, his father had driven away everybody else, his mother, his sister, his brothers.

So Harry listened, and after his father died he locked this morbid stuff away in his heart, and tried to imagine it had gone away.

But it had not.

The January morning was still grey when Harry Wooler walked into London from the north, passing through the wall at Newgate. It was here that cattle, sheep, pigs and chickens were funnelled into the city to the shouts of the drovers, a steady flow of provision pouring into an ever-hungry gullet. It was like walking into one immense farmyard, Harry thought. Further south he came to the slaughterhouse district where the animals were killed, skinned, and dismembered, and then continued their journey in bits to the butchers' shops and the tanners'. Here he found himself walking on a slick of blood and animal guts, steaming in the cold air, and there was an almighty stink of shit and piss, and the iron tang of blood.

Then he pushed on south to Cheapside, where the farmyard bleats were drowned out by the clank of metal on anvil and the pounding of nails into wood or leather, as the blacksmiths, goldsmiths, silversmiths, tanners, dyers and potters all laboured, and the cowshed stink was replaced by the stench of the fullers' urine jars. Cheapside was a magnificent, unending festival of trade. You could buy anything you wanted here, from a hot veal pie to a flagon of beer, from a Flemish-style hat to Italian-style shoes, from imports like French linen and Spanish silk to eastern spices and Scandinavian walrus ivory – from the words of an apocalyptic preacher that would fire your soul, to the moist quim of a girl to soothe your body. For Harry this was a wonderful place to be, for all the stinks and the filth and the crowding, and the beggars that swarmed like crows pleading for spare farthings.

Harry was Oxford born and bred, but he was a merchant, and England's capital of business felt like a second home to him. And in Cheapside industry and commerce pulsed as nowhere else in England.

But today Harry was not here for trade.

He pushed further south still, past the walls of Saint Paul's, and through narrowing streets lined with warehouses. Their walls were plastered with posters bearing apocalyptic pronouncements from the Bible. He looked at the posters curiously, for they were printed, a novelty still rare in Oxford.

He came at last to the river bank. He could just make out the brown, filthy water through a forest of cranes and ships' masts and furled sails, and he could hear the cursing of the stevedores in a dozen languages. He was near the old bridge, the only span over the river to the south bank. To his left there was the ugly pile of the Tower, to his right the great palace of Westminster with the abbey beyond, in their suburb around the bend of the river. He turned right and walked perhaps a quarter of a mile along the bank. When he came to the ancient dock road called the Strand he cut north, heading inland past more warehouses and factories.

And here he found, just where the monk's letter had described, a small, gloomy parish church, its roughly cut stone stained black by city soot. Chantries clustered around the church, chapels devoted to the souls of the long-dead rich. A stone tablet told him that the church was dedicated to Saint Agnes, a virginal martyr of Rome, and his sister's patron saint.

He felt a deep reluctance to enter. But it was to here that he had been summoned.

The letter from the monk had arrived in a pile of business correspondence. At his home in Oxford, Harry Wooler had read the letter, from a Carthusian called Geoffrey Cotesford, with a sinking heart, for it concerned a matter of conscience. Harry didn't regard himself as a sinful man, but he preferred to stick to commerce, and leave the affairs of the soul to others. But he could scarcely ignore this summons, for it had concerned his sister, Agnes, lost since Harry was a boy.

The church's heavy wooden door was open. Harry stepped inside. The church was cold, its heavy stone walls sucking out the heat, and the air was thick with incense. A man swept the floor with a cane broom, a portly fellow in a loose black robe, but otherwise the church was empty. Harry knelt at a pew, crossed himself, and uttered brief prayers. Then he walked up the church's central aisle to the altar.

He paused by an elaborate tomb that had been set against one wall, cut from some black stone. It was on two levels. Above rested the figure of a man of about fifty, handsome in life, well-dressed in a robe like a Roman toga, with his hands clasped in prayer. But on the level below lay the same man given up to decay, his clothes rotted to rags, his skin peeled back to reveal a cage of ribs, those praying fingers reduced to bone.

Harry didn't like transi tombs. The hideously realistic corpses always reminded him of his own father's death, and his final morbid mutterings.

But Harry was twenty-five now. He was a merchant, as his father had been before him, and his grandfather too – his family as far back as anybody cared to mention. As their name implied, the Woolers sold prime wool from the heart of England to the continent. Trade was what interested Harry – trade, and stories of exploration, of Prince Henry and his school of navigation in Portugal, of new routes to India and China and perhaps even to countries nobody had even heard of yet. Harry didn't like transi tombs and chantries. He didn't even much like churches, he admitted. Give him Cheapside any day, rather than this!