Ammanius said, 'The British lost their land, footstep by footstep. So here they are, refugees fleeing from the land of their ancestors. And in the last decade Reptacaestir has been trodden by the feet of a new wave of invaders.'

'What do you mean?' Wuffa asked.

The bishop brought them to a small church, constructed of bits of Roman stone. 'This is a chapel dedicated to Augustine. Just ten years ago the archbishop landed here, with a mission from the Pope to convert you heathen children to the one true faith. And this is one invasion of Britain which will know no ending.'

Wuffa looked around at the battered walls, the swarming Norse and German traders, the huddles of British refugees. Standing amid these complicated, many-layered ruins, he sensed the past, as if the doors of a vast abandoned hall opened to him. It was thrilling, disturbing.

And yet when he glanced at Sulpicia it was only the bright present that filled his mind, like the diffuse light off the sea which banished the shadows of the fort's rotting walls.

Wuffa and Ulf spent some days escorting Ammanius to other south-east ports, where the bishop had to supervise more bands of refugees fleeing to the continent. Many of the ports had massive old Roman fortifications like Reptacaestir's. The only one Wuffa had heard of was Pefensae, which the bishop called Anderida. Here, after the Romans, a British town had grown up within the walls, but a century ago the Saxons had landed here and slaughtered every last Briton, a bold strike of which the scops still sang.

With Ammanius's obligations fulfilled, the six of them set off for the far north, in search of the legend of Isolde.

Their journey mostly followed the roads left behind by the Romans, some of which were well maintained, some not. Ulf and Wuffa travelled by horseback, while the bishop, Sulpicia and the novices rode in a sturdy Saxon cart. Britain was full of petty kingdoms, but Ammanius was able to transfer them from the protection of one polity to the next through letters he carried from his archbishop – and, Wuffa thought, by his own sheer force of personality.

By night they stayed in old Roman towns, or in forts on top of hills, or in villas in the countryside. Ringed by hastily built walls the towns were more like shabby fortresses, where amid thatched houses of mud and straw a few mighty stone structures loomed. In the Roman-British domains the towns were protection in bad times, markets in good times, and places where kings or other petty rulers collected their taxes.

The forts on the hills were more interesting to Wuffa, because they were so different from anything he had seen before. They were fortified not by stone walls like the towns but by earthen ramparts and wooden palisades. Ammanius, aware of Wuffa's growing curiosity, told him these forts had brooded on their hills long before the Caesars ever came. 'And later the British drifted back to the fortresses of their ancestors. It was as if the Romans had never been here at all…'

Ammanius preferred to stay in the villas. Grand old farmhouses, once owned by rich Roman British, they had been either abandoned when the Roman system broke down, or occupied as much-reduced farms. And later, as Britain's Christianity spread, they became monasteries.

Here, surrounded by the calm toil of monks, Bishop Ammanius evidently felt at ease. And as he relaxed he drank. Holy man he might be, but Ammanius was fond of his wine.

And the drunker he got, the more fascinated he seemed by Ulf and Wuffa. Ammanius spoke more often to Wuffa. He said he saw the 'empty minds of two pagan boys' as vessels to be filled up with his God's truth. But when the big Norse moved the bishop's stare always followed, as if Ulf were some fascinating animal.

One long evening the four of them sat in a firelit room deep within a windswept monastery-villa. They were alone save for a novice who brought them food and drink. Tapestries hung on the walls and there was a thick carpet on the floor. This had been the triclinium of the Roman villa, the bishop said, a word that meant nothing to Wuffa; evidently it meant some kind of living room. The monks said that the carpets and tapestries were there to keep the pagan symbols on the walls and floor from pious gazes, and also to warm a room whose system of under-floor heating had long since broken down.

Ulf and Sulpicia played a complicated game of dice and counters, worn with use, left behind by the villa's original owners and now popular with the novices. Sulpicia sat on her couch close to Wuffa, her loose tunic falling around the soft flesh of her neck. Wuffa was aware of every soft laugh she and Ulf shared, the way Ulf's tousled golden hair touched her dark British brow, the way their fingers touched over the grimy surface of the wooden game board.

Since that pivotal day in Lunden when they had met, Wuffa had believed he had had an agreement with Ulf, that Sulpicia was, if not Wuffa's, at least his to try for first. But was Ulf to be trusted? Was he more subtle than Wuffa, was he quietly working to take the advantage? Wuffa felt baffled, out-thought.

And if Wuffa watched Sulpicia, so Ammanius watched Ulf.

Ammanius leaned close to Wuffa, and the Saxon could smell the stale wine on his breath. 'You Germans fascinate me,' he said. 'You don't build empires. You have no values save loyalty to your chieftain's hall, where your warlords sit around and get drunk. You have no laws, save the most brutal. You actually put a cost on a man's life, don't you? A penalty to be paid if one takes it?'

'We call it the wergild.'

'Nothing but a rationalisation of a barbarian's blood-feud. And you enforce your laws by maiming, by mutilating eyes and tongues and limbs. I've seen the results! Your society is riddled with violence; it is run by it. You have no medicine to speak of; the sick, the handicapped, the old, you put to death.'

'Don't believe all you hear about us from our enemies,' Wuffa said evenly.

'Even your religion is only a ragged collection of myths and legends.

Your stories of Woden, your earth-mother Frig… By Christ's eyes.' He took another long draught from his wine cup, which a nervous-looking novice refilled. 'And yet,' Ammanius said, his chin stained red by spilled wine, 'and yet you have much to envy. Oh, yes! The passion of a warrior people, the primitive vigour. Your guttural tongue is full of words for "love", for "honour" – so unlike the cold formality of Latin-' He belched, leaned further, and tumbled off his couch, landing heavily on the carpeted floor.

The novice came hurrying over, a resigned look on his face. Wuffa and the novice took an armpit each, hauled the bishop heavily to his feet, and began to lead him from the room.

'The love between warriors,' Ammanius cried. 'The bond between strong men! Is there such a bond between you and your Norse, Wuffa?

…' But he was gulping, and they only just managed to get him out of the door before he vomited heavily, spilling wine-dark bile over the carpeted floor.

Ulf and Sulpicia hadn't said a word through this exchange. They continued their game, the worn pieces tapping across the antique board.