The next day the travellers moved on, heading steadily north. Bishop Ammanius was poor company, glowering at the world, still stinking of vomit and drink, and taking out his anger on the hapless novices. They were all locked together by unspoken lust and burning jealousy, Wuffa thought.

They reached at last what had once been the northernmost province of Britannia, which Ammanius called Flavia Caesariensis, and they made for the principal town, Eoforwic – Eburacum, as the Roman British had called it. This turned out to be a spectacular Roman city, set inside massive walls on high ground overlooking a river. It was dominated by a grand stone building, its tiled roof and colonnades intact. This had been the headquarters of the old Roman fort, Ammanius said, the principia.

But as the travellers approached Wuffa saw that the city walls were breached and burned. Inside the town there was much activity, with the walls being repaired and traders and immigrants moving in. These busy folk were not Romans, or British. Eburacum was in the hands of Germans now.

When Roman authority withdrew, a Roman military commander called the Dux Britanniarum had used this legionary capital and the forts on the Wall to take control of the old northern province. The polity had survived well, despite raids on the east coast, where over the decades a German people known as Angles had landed in great waves. For a time the British had confined the Angles to a coastal fortress called Bebbanburh, and pushed them back still further to an offshore island called Lindisfarena. But the Angles kept coming, and had long since broken out. Now their kingdom sprawled across the north of Britain, and in just the last few years they had taken Eoforwic for themselves.

And today, cattle were herded beneath the colonnade of the principia, and German chieftains stalked over its marble floor. Ammanius, surveying all this, tried to convey to a reluctant Wuffa his sense of loss, of regret, a feeling that he had been born out of his time.

They stayed in the city only one night, before travelling on to the centre of the new Anglish kingdom on the east coast. Bebbanburh was a stronghold built on to a plug of hard black rock that loomed uncompromisingly above a bank of dunes. They had to climb stairs cut into the rock to reach its summit. The stronghold was crude, only a handful of wooden-framed huts surrounded by a hedge. Once this slab of rock had been the whole of the Angles' holding. Now it was the heart of a kingdom that sprawled across northern Britain.

It was named after the wife of an Angle king. The British had once called it Dinguardi, but nobody cared about that.

The weary travellers were greeted by a thegn of the local king, and were granted lodging in a small, cramped hall. In this typically Germanic building Wuffa felt more at home than since he had left Coenred's village. It was a spectacular site too, looming above a restless sea over which the comet spread its ghostly light. But the bishop was soon in a black mood, for as he pressed the king's advisors for news of how he could track down Isolde's prophecy he was told there was yet more travelling to be done – and this time west, along the line of the old Roman Wall itself. 'The Last Roman', the thegn said superstitiously, said to be a descendant of Isolde herself, was to be found haunting a Wall fort called Banna.

Wuffa, indifferent, found himself a corner to curl up on straw that smelled of cattle, and fell soundly asleep.

He was woken in the pitch dark by a heavy, wine-soaked breath, a clumsy hand fumbling beneath his blanket. Without thinking about it he raised his knee, jammed it into a fat belly, and lashed out with his fist. Ammanius fell back with a grunt; of course it was him.

Furious, Wuffa scrambled up from his straw pallet, went to the door and kicked it open. By the comet's light he could see the bishop sprawled on his back, a dark bloodstain spreading over his tunic. 'In the name of your God nailed to His tree, what are you doing, Ammanius?'

The bishop pawed at his face. His words were muffled, masked by the gurgling of blood. 'I think you've broken my nose.'

'I should have broken your drunken neck. Why did you come to my bed?'

'Because,' the bishop said desolately, 'she was in his.'

It took Wuffa, still dizzy from broken sleep and shock, some time to work out what had happened. The bishop, perhaps misled by signals from Ulf that may have existed only inside his head, had gone to the Norse's bed – and there he had found Sulpicia. He had come to Wuffa out of desperation and longing.

So, Wuffa thought bleakly, in one gruesome moment the tensions that had been building up between the four of them all this long journey had come to a head. He ought to feel anger, but he was too numb for that. He gazed out of the doorway, at the comet which sailed over the ocean.

The bishop floundered on the floor like a beached fish. 'We are betrayed, Wuffa, both of us! Betrayed!'

They had to ride south to the line of the Wall; coming up along the coast they had bypassed the old fortification. They passed through a gate fortress, unmanned, long abandoned and derelict. Then they came to a road in reasonably good repair that ran along the south face of the Wall, beside the track of a rubbish-filled earthwork. They rode along this road, following the line of the Wall west towards Banna.

The Wall showed its age. Its clean-cut facing stone had been robbed in places to expose a rougher core of rubble and cement, but there were long stretches where it survived, and even traces of whitewash and red paint that must have been centuries old. The gate forts and turrets were regularly spaced out, and from higher ground you could see them like distance markers along the Wall's line. There were more major forts too, nuzzling against the line of the Wall: 'forts' that were the size of small towns. Some were still occupied, no longer by soldiers but by farmers, some British, some German, dwelling in humble wooden halls that huddled in the lee of the great structures of the past.

And as they rode, gradually the sheer scale of the Wall impressed itself on Wuffa's mind. The Wall simply cut across the countryside, allowing neither ridge nor river to stand in its way. Spanning the neck of this island country from east to west, from coast to coast, it enclosed the entire southern portion of the island, from Eoforwic to Lundenwic to Reptacaestir, protecting all those fragile places from the predations of the barbarians who had lived in the further north. And for all its decrepitude it was so immense it took them four days to ride its length. Wuffa had never been one to gape in awe at ruins. But as he grew to understand the Wall he felt he glimpsed the towering, inhuman ambitions of emperors who with a single decree could order a country cut in half.

And in the shadow of the mighty Wall the four of them were still mired in rivalry and lust.

Since Bebbanburh any friendship Wuffa had had with Ulf had been corroded by envy. Ulf had come to seem sly to Wuffa, manipulative and false – and he had won Sulpicia, which maddened Wuffa. Sulpicia herself seemed offended by Wuffa's anger. As far as she was concerned she belonged to herself, and was not some slave to be fought over.

But as the journey continued her health worsened. She tried to hide this, but Wuffa saw her holding her belly, and heard her emptying her guts in the mornings. Had Ulf planted his Norse seed in her? If he had, it did not make her happy. Wuffa didn't imagine her people would welcome her back with a barbarian's brat at her tit.

And Ulf backed away from her. Now he had won her, now she was ill, he showed no interest in Sulpicia. His coldness infuriated Wuffa even more. He would not behave this way if the child were of his loins, if Sulpicia were his.