He saw something move. Across the street, beyond a low wall: large, heavy, a flash of golden hair. Without thinking Wuffa pivoted and fired off the cobble. He heard a satisfying thud of rock on flesh.

'Ow!' His target straightened up. It was a man, dressed in a leather tunic and trousers, with a ragged shock of blond hair. He was carrying some kind of spade. And he was clutching his balls. He glared at Wuffa and began striding across the road. He was big, with muscles that bulged through his sleeves. He spat abuse in a Norse tongue of which Wuffa could make out only one word: 'Arsewipe'.

Wuffa was a Saxon warrior, son of Coenred, and he stood his ground, his hand hovering at the hilt of his seax, his bone-handled knife.

The big Norse came to a halt not an arm's length from Wuffa. The Norse was about Wuffa's age, around twenty, and they were both blond and fair-skinned, and dressed similarly in leather tunic and trousers. But Wuffa wore his hair in the Saxon style, shaved at his forehead and long at the back of his neck, while the Norse wore his yellow mane loose and ragged.

Wuffa recognised this man. 'I know you,' he said in his own tongue. 'You're from the fleet at the wharf.'

The Norse fired out more insults.

Wuffa tried again, in Latin. 'I know you.'

At least that stopped the flow of abuse. 'So what, arsewipe?'

Britain was an island populated by Roman-British, Germans and Irish, with traders always streaming over from the continent. Most adults knew a little Latin, a relic of empire, the only common tongue. This young Norse was no exception. Though he evidently didn't know the Latin for 'arsewipe'.

'I am Coenred's son. We are unloading your boats-'

The Norse kicked a loose rock. 'And is this how you greet your trading partners, with a cobble in the balls?'

Wuffa held his gaze. They both knew they had a choice here, either to fight it out or resolve their difference. Wuffa said, 'I should be working. Even if you don't kill me my father will finish me off for you.'

The Norse laughed. But he warned, 'You have to say it.'

'All right. I apologise.'

The Norse grunted. 'Very well. Your little girl's throw did not harm me anyhow.'

Thus the matter was closed.

'I am Wuffa, son of Coenred.'

The Norse nodded. 'Ulf, son of Ulf.' He squinted up at the wall. 'If you are not hunting Norsemen's balls, what are you doing?'

'Smashing windows,' Wuffa said, a bit ashamed. He hefted his sling. 'It helps improve my aim.'

'Of course it does.'

'And you?'

Ulf showed him his spade. 'Looking for coins. Sometimes the Britons bury their treasures, in the hope of returning some day.'

'They never do.'

'And if they did they would be disappointed, for Ulf the treasure-seeker has been there before them. Well, arsewipe. Do you want to continue throwing stones like a baby or will you help me dig?'

There was a kindred spirit here, Wuffa thought. He pocketed his sling. 'Let's dig. But stop calling me "arsewipe". How do you know where to look?…'

Ulf held up his hand. 'Hush. Can you hear that?'

It was singing, voices joined in a melody high and clear as the sky, drifting on the afternoon breeze.

The young men exchanged a glance. Postponing their treasure hunt they set off across the broken city, curious, ambitious, unperturbed by the monumental ruins around them, living in their own present.

They travelled across the city, heading towards the ruined fortress built into the south-eastern comer of the circuit of the city walls. The singing came from a massive stone-walled building with a roof of red tiles that stood not far from the walls. Its huge wooden doors were flung open, and the setting sun cast low light into long aisles.

A group of people had gathered before the open doors. Men, women and children, there must have been four, five hundred of them, Wuffa thought. Drawn up in a loose column they had begun walking slowly along the road down towards the docks. A man in colourful vestments led them; he wore a pointed hat and carried what looked like a shepherd's crook. The column was flanked by parties of Saxons – warriors, evidently hired to protect the pilgrims. The Saxons talked amongst themselves, chewed on bits of root, and eyed up the prettier women.

The pilgrims were Britons; Wuffa could tell from their clothes and hair. The men all wore their hair short and clean-shaven. The women mostly had their hair in neat plaits and buns. Men and women alike wore sleeveless tunics under cloaks, and were adorned with bracelets and armlets and necklaces. One or two of the men even wore togas, long swathes of cloth that scraped across the dusty ground. But they were mostly dressed for travel, and laden with baggage. Even infants old enough to walk bore bundles on their backs and heads. They looked strained, unhappy, fearful and uncertain.

They were all Christians, most likely. In among them were clergy wearing tonsures cut in the British style, with the front of the head shaved from ear to ear and the hair worn long down the back. That man who led them, though, wore a Roman tonsure, with the crown of his head shaved in a disc.

And as they walked the pilgrims sang, creating a chilling, unearthly music that rose up to the sky, where the hairy star shone ever brighter.

Ulf gaped at all this. 'So what's the big building? A warehouse?'

'No. It's a church. They call it a cathedral.' The cathedral was younger than the city. It was built of reused stone; in places where the facing stone had fallen way you could see bits of pillars and statues broken up and used as core. But the reused roof tiles were cracked, the glass in the windows smashed. Nothing was new here, Wuffa thought; there were only degrees of age.

Ulf asked, 'Was this big church built by your great king?'

'No, Aethelberht's church is over there.' Wuffa pointed north.

'Why do you need two churches?'

'The King follows Roman Christianity. He was converted by Augustine's bishops. This church was built by British Christians.'

Ulf thought that over. 'I'm more confused.'

'The walkers are all British Christians. I think. The man leading them is a Roman, a bishop.'

'So why are they following him, if he's not one of theirs?'

'I-' Wuffa spread his hands. He knew next to nothing about Christians. He only observed their behaviour from outside, as if they were exotic birds. 'They are leaving for good. You see it all the time. Look.' Wuffa pointed. 'See the jewellery? They are wearing their wealth. These are the people who bury your coin hoards. Their church is organising the flight.'

'Where are they going?'

'To the west, perhaps, or over the sea to Gaul.'

'Away from you Saxons.'

Wuffa grinned. 'Away from us, yes.'

'Carrying all that wealth makes them easy to rob.'

They shared another glance. But then they turned away, the thought unfinished. Evidently, Wuffa thought, neither of them was an instinctive thief.

A bonfire burned on the road, and the hymn-singers had to divert to pass it by. An abandoned house was being looted by a pair of Saxons, a rougher sort than the mercenary warriors who accompanied the refugees. The looters evidently weren't having much luck. They hurled old clothes and broken furniture out of the house and onto the fire – and books, rolled-up scrolls of parchment and scraped leather and heaps of wooden leaves that curled and popped as they blackened. Most of the pilgrims passed by this scene with eyes averted.

But one old man, his toga flapping about his skinny frame, broke from the column and tried to get the books off the Saxons. His cries were a broken mixture of British and Latin: 'Oh, you pagan brutes, you illiterate barbarians, must you even destroy our books?' A young woman called him back, but friends held her.