'But a church of free men and women cannot be controlled by the centre, and so it won't do for an imperial cult,' Nennius said gloomily. 'Perhaps in future the Roman church will deal with all Pelagians as it has with Pelagius himself…Think of it, Ambrosius! Excommunicated for proclaiming that mankind enjoys free will! What is the world coming to? Where is our church going?'

'The question is, where are you going, my friend?' Ambrosius asked with gentle humour, pouring more beer. 'Are you still determined to find your cousin on the Wall? It won't be an easy journey.'

'No. But as I told you in my letters it is a dream of freedom that draws me there. Pelagius would approve! A dream, or rather a hint, a tantalising hint of a better future…' And he began to speak of family legend and of fragmentary prophecies, of emperors and of history.

But the evening was ending for Isolde. Exhausted by travelling, heavy with food and drink, and with the baby slumbering softly within her, she made her excuses and left for her bed. The old men talked on softly, as young Damon, huddled by the fire, listened as attentively as a puppy before its master.

For Isolde the journey north to the Wall was a long and brutal haul along the spine of this dismal island. Following poor roads they passed through town after shabby walled town, and they had to pay more tolls as they crossed invisible provincial boundaries.

Isolde was so immersed in the oceanic aches of her own body she barely noticed the change in the character of the country as they headed north, from the rolling chalk hills of the south with their abandoned farms and fortified country houses, to the more rugged north with its bristling forts. But the further north you went the fewer Saxons you saw. Perhaps the people in the north had found other ways to look after themselves.

They arrived at last at the line of the Wall. Though the paintwork was faded, and in places you could see where damage had been roughly repaired, the Wall was still intact and very impressive, its powerful lines cutting across the neck of the countryside.

And the Wall was manned. Nennius turned westward, planning to travel to a fort called Banna. They soon reached what Nennius called a mile-fort, built around a gate that was roughly blocked with stones. Two grubby soldiers in woollen tunics flagged them down, to extract still more tolls. According to the soldiers the whole line of the Wall was under the command of the 'Duke of the Britains'. The soldiers gave them a chit scribbled on a wood slip, so they wouldn't have to pay any further tolls.

Isolde found the Wall and its soldiers, even the process of paying the toll, reassuringly familiar. Sooner this semblance of the Roman way than relying for your protection on the bands of blond-haired barbarian thugs like in the south. But no standard was erected over the mile-fort's eroded stones; no eagles flew here.

They continued westward, passing more mile-forts and watchtowers, and arrived at Banna. The fort sprawled on an impressive escarpment, and to the south a river with shining gravel banks curled through woodland. The northern wall of the fort was built into the line of the Wall itself. There were houses and other buildings outside the walls of the fort, but they looked abandoned, their roofs missing, walls of mud subsiding back into the earth.

They passed through a gateway in the eastern wall. The soldiers on duty let them pass with a brisk inspection of the chit from the mile-fort, and a letter of passage Nennius carried from his cousin Tarcho, the commander here. The fort was crowded and busy, with men, women and children going about their business. The civilian settlement outside might have been abandoned, but the people hadn't gone away, they had just moved inside the fort's walls. And the soldiers were still here, evidently.

Isolde recognised a granary, its floor raised for ventilation, a second granary which looked abandoned, and blocky buildings which might be the fort's headquarters. Some of the buildings were quite impressive, large and stone-built. But many were derelict, their roofs collapsed, their walls robbed of stone.

Nennius was excited to be here. Once their remote ancestors had lived here, he said. He knew that because his grandfather, Audax, had told him that the famous Prophecy had actually been created here at Banna. But there was no sign of that lost primeval home in this decaying fort, and even Nennius's nostalgic enthusiasm soon faded.

To Isolde's surprise, they were led to the intact granary. As they walked inside Isolde realised that it had been converted into a hall, its interior divided up by wooden partitions. But there was still an agricultural smell about the place, Isolde thought, the dry tang of the grain that had once been piled up here to feed hundreds of long-dead soldiers.

They were greeted by Tarcho, Nennius's cousin and commander of Banna, and by his wife, Maria. Evidently about the same age as Nennius, in his fifties, Tarcho was a big, slightly plump man with a bristling moustache, and his hair was a pale strawberry-blond laced with grey. He wore the insignia of a Roman soldier, including a handsome officer's belt, but also a shoulder-brooch and a belt heavy with knives, like a Saxon. His wife, too, a plump ball of energy and bustle, wore silver sleeve-clasps, Isolde noticed with faint envy. The Saxons hadn't come this far in great numbers, but their fashions had, it seemed.

Nennius greeted Tarcho eagerly. For him the end of a long quest was nearing. For Isolde, though, it was just another day of her pregnancy, and a long, hard day at that.

Maria saw this and immediately took Isolde under her wing. 'Oh, my dear, I know exactly how you are feeling. I should, I had five of my own, all boys, all of them as fat as their father, and look at him. Come,' she said, taking Isolde's arm, 'let's see if we can make you comfortable in this soldiers' hovel…' She led Isolde to a small private room with a couch and pillows, and brought her hot water in a bowl. Her palm was rough, her grip strong, her skin dry, a worker's hand. 'I know you're far from home,' Maria said, 'and you must be frightened. But your father and my husband are cousins, so you're with family, aren't you? And believe me you're better off here than anywhere else. The soldiers always did have the best doctors. You'll be in good hands, I promise.'

'Thank you,' Isolde said sincerely. It was a huge relief not to be totally dependent on her father.

It was already late afternoon. She lay on her cot and slept a while, to gather her strength before the evening meal.

That evening Isolde was the last to join the group. They were in the largest room in the granary-hall, set out like a Roman triclinium with couches around a small central table. Nennius was holding forth about politics in Rome. He had a small leather satchel on the table before him. Isolde knew it contained documents about the purpose of his quest-to retrieve the Prophecy, as he called it, lost so long ago.

Her belly feeling heavier than ever, Isolde levered herself down onto a couch. The light from lamps and candles was cheerful enough, and the food, meat, bread and stewed vegetables, was warming and palatable, if it lacked spices for Isolde's taste.

This corner of the old granary was walled on two sides by unplastered stone through which holes had been roughly knocked to make windows. It was a working room, an office of sorts, with desks heaped with scrolls and tablets. Christian symbols could be seen in the clutter on the desks-a bronze fish, a chi-rho medallion. And the papers on Tarcho's desk were weighted by a stone statue, a reclining woman painted crudely in blue, the colour of the Virgin Mary. Isolde learned later it was a much older piece, a Roman soldier's carving of a local goddess called Coventina of whom nobody remembered anything but her name, now repainted as Christ's mother.