But if the Germans' religion was evident, so was their brutal justice. From a gibbet hung a desiccated corpse, upside down, suspended by its ankles. Macson said that this method of execution was particularly favoured by King Offa, who had used it on many of his own unruly relatives. It must have been a slow and gruesome way to die.

And they passed abandoned Roman towns, where the fire-scorched ruins of offices and shops and bath-houses rose out of choked greenery. The British had sustained these towns long after the withdrawal of formal Roman rule, but the German immigrants had shunned them, preferring their own architectures of wood and earth. Indeed in some places the Germans had killed the towns, by stopping up their wells with rubble. To Belisarius, who had grown up among the enduring splendours of Constantinople, these bowls of rubble were a poignant sight. What a fragile barque was civilisation, battered by paganism, ignorance and plague.

Macson said, 'The Germans have a notion they call wyrd. Like fate – but vaguer, more entangling. They believe the Romans were brought low because they had desecrated the god-throttled landscape, because it was their time to go – because of wyrd. Now the Germans are building their own kingdoms. But they believe they must live well, or wyrd will do for them in their turn.'

'I thought the Germans were Christians now. What are they doing entertaining such pagan ideas?'

Macson snorted his contempt of all things German.

Fearful of wyrd or not, the Germans had a vision of their own, as Belisarius learned when they came upon Offa's Dyke. This was an earthwork wall, four or five times a man's height, topped by a wooden palisade or in places by stone breastwork. Some of its towers and gates were manned by tough-looking warriors in mail coats, and signal beacons flickered.

And this wall was a hundred Roman miles long, a mighty fortification that ran from the estuary of the Sabrina in the south to the coast in the north, terminating at a settlement of the wealisc called Prestatyn – although there were stretches, Macson said, where it incorporated a river and older fortifications. The Dyke was a north-south barrier erected at the orders of Offa to separate his German kingdoms from the lands of the wealisc, the expelled Britons, to the west, and thereby to stabilise a troublesome border. Only a few years old, the Dyke was fresh and raw, a wound slashed through the flesh of the green British countryside.

After some days on the road, the old man, Caradwc, seemed to brighten. Belisarius wondered if the clean air away from the muddy German towns was doing him good. He began to make conversation in passable Latin. He asked about holy sites Belisarius had visited, relics he had seen in the course of his travels. 'And tell me, when will the Emperor return to this province and sort out these heathen Germans?'

Long after the fall of the empire in the west, the emperors had nursed ambitions to regain the lost western provinces. Roman trading ships, bringing goods with which to court the British leaders, had been dispatched as part of a long-term strategy to woo back Britannia. But time had passed. New barbarian states sprouted in the ruins of the old western empire, which became an increasingly distant memory. And in the east the empire was battered by new pressures, notably a new enemy: the Saracens, the warriors of the new religion of Islam.

'When I was a young man the emperor was Constantine the Fifth,' Belisarius said. 'What a warrior! He scored victories against the Saracens and the Bulgars alike. My own father served with him. It was a golden age. In the end Constantine was succeeded by a sixth Constantine, then a boy of ten, who is run as a puppet by his mother the regent. Even now people gather at the tomb of Constantine, I mean the fifth, calling on him to return and lead us. But we will never see his like again…'

'But,' Caradwc pressed, gripping his arm, 'do the emperors not still dream of Britain?'

Belisarius gently extracted his arm, disturbed by Caradwc's anachronistic longing. 'I'm afraid most of us don't even know where Britain is, old man.'

Caradwc seemed unreasonably disappointed by his answer.

Perhaps what really distressed these British, who still thought of themselves as Roman, was that Offa's Dyke was a frontier barrier just as the Romans had once built, but now intended to exclude them, the new barbarians.

In the north, the character of the country changed. The chalky fields and rounded hills of the south gave way to a harsher landscape of mountains and valleys that looked as if they had been gouged out by some vast, vanished force, and on some of the higher moorland Belisarius saw huge boulders, obviously out of place. How had they got there? Perhaps this was the legacy of the Flood, the country a vast wreck through which humans crawled like crabs in the hull of a beached ship. It was no wonder that the imperial Romans had always failed to tame this rugged landscape.

They reached the grand old Roman fortification which everybody simply called 'the Wall'.

Even in a ruinous state the Wall ran like a stone seam across the countryside. Caradwc fitfully told him of what the Britons remembered of the Wall's construction: it had been built, he said, by Romans at the request of the British after the collapse of the imperial province. That seemed unlikely to Belisarius, but the truth was, four centuries after Britannia, nobody knew any more. It was a remarkable relic, even to a man from Constantinople – but somehow Belisarius found it less impressive than Offa's Dyke, perhaps because that cruder construction was of the new age, whereas this mighty ruin was of the past.

Heading for the coast, they turned east and followed a road that ran along the line of the Wall on its south side. In this hilly northern country it was unseasonably cold and damp, and after a few days Caradwc, never strong, sickened again.

For some days they were forced to make a rough camp in the shelter of the stone walls of an abandoned fort called Banna. While Caradwc was sleeping Belisarius explored the ruins, which were perched on a crest high over a valley cut by a winding river.

Macson joined him, and they talked beside a desultory fire. Macson seemed nervous now that their goal, the isle of Lindisfarena, was only a few days away. He sat upright, his muscles hard, one foot tapping restlessly at the ground. 'I apologise for the delay,' he said.

'You can't help your father's illness. But that isn't the reason you're so tense, is it? I'm well aware that there is much you haven't told me, Macson. You're after more than just helping me sell a few books to the monks. You have an ambition of your own, something you want to achieve at Lindisfarena. Isn't that true? And in your meeting with me you saw a chance of achieving it.'

Macson grunted. 'Chance? If a man keeps pushing at a locked door until it falls open, would you call that chance? Yes, there is something I want at Lindisfarena. But perhaps there is a way you can profit too, Belisarius.'

He told Belisarius the story of Sulpicia, his ancestress, of how she had come to the north – 'somewhere along the line of the Wall, nobody remembers where, perhaps it was here' – and found herself caught up in a dispute between a Northman and a German over a strange document called 'the Menologium of Isolde'.

'You must remember that I am recounting family legends preserved by slaves – illiterate slaves at that. This Menologium was a prophecy of some kind. It belonged to an aged Briton. And it had already begun to prove itself, already come true. That's the crucial thing. Now, the Northman and the German fought. The Northman killed the old man, or it may have been the German, and the German raped Sulpicia, leaving her pregnant, or it may have been the Northman. Between them they stole the prophecy, and Sulpicia, abandoned and pregnant, left ruined, was forced to sell herself and her unborn child into slavery.'