The violence that simmered affected everybody. Wuffa and Ulf even came to blows once, over a trivial argument about the best way to ford a river by a ruined Roman bridge.

In the end Ammanius took Wuffa and Ulf aside. 'I hired you two for your muscle, but I scarcely expected you to turn on each other. Remember you are in my pay. Try to think with your heads, not your cocks.'

However it was the bishop himself who had contributed most to the group's tension. With his battered nose bloody and sore, he raged at the novices, at Wuffa and Ulf, even the horses when they shied. Wuffa saw that Ammanius's anger was really for himself, for the way he had behaved that night at Bebbanburh. But he was a prisoner of his own flaws, as all men were, Wuffa thought.

Thus the little group, barely speaking, at last approached Banna. Here, not far from its western end, the Wall strode over a high ridge from which Wuffa could make out the hill country to the north, and to the south a river wound through a deep wooded valley.

A small, mean village of Anglish farmers huddled a little way away from the fort, down the northern slope. On arrival, Ammanius led his party to the village, fearlessly summoned the chieftain, and demanded to know if the man knew anything of this 'Last Roman'. Wuffa and Ulf had haltingly to translate for him, for these Anglish knew no Latin, and Ammanius certainly knew no Germanic.

Yes, said the Anglish farmer-warrior, he knew all about Ambrosias, the Last Roman. In fact he and his people had been keeping the old man alive for years.

The Anglish had been encouraged to settle here by their kings. They had chosen not to live inside the old fort, but they would rummage there for abandoned tools, coins, even bits of jewellery, the detritus of centuries.

And in Banna they had found Ambrosias. For generations the old man's family had lived in the township that had grown up inside the ruined fort. With the coming of the Anglish his family had all packed up and gone, the farmer neither knew nor cared where. But the old man, stubborn, had remained alone, scraping at the dirt of a small-holding inside the walls of the fort. He was magnificent, in his frail way. He had even raised his rusty hand-plough and had threatened to break the heads of any burly Anglish who tried to evict him from his fort.

Some impulse led the Angles to tolerate the old man. They even shared their ale with him. Ammanius, hearing this, complimented the farmer on a Christian generosity surprising in a 'hairy-arsed heathen'. But Wuffa knew it was easy to be awed by the Romans' mighty ruins. Perhaps to the Anglish, some of them newly arrived from across the sea, the old man of the Wall had seemed like a relic of vanished days, a living ghost. They may even have been trying to propitiate the gods of the Wall by keeping him alive.

But it had been fifteen years now, the burly farmers grumbled, and still the old man refused to die.

They rode into the fort. Choked by grass and weeds the place was very old. Halls of wood and wattle had been erected on the neat rectangular foundations of vanished stone buildings, but even these latter huts had slumped back into the dirt from which they had been shaped. But the place was not quite abandoned.

Ambrosias was gaunt, perhaps seventy years old, and wore a thick, hooded woollen cloak even though the spring weather was not cold. But he wore his silver-grey hair cut short, and he was clean-shaven, though his leathery skin was stubbly. He must once have been handsome, Wuffa thought, with a proud nose and a strong chin. Now, though, his face looked sunken in on itself, and his frame was withered.

This was the 'Last Roman', kept alive as a sort of pet by illiterate Anglish farmers.

And when Ammanius approached him, Ambrosias ignored the bishop and turned to Ulf and Wuffa. He was avid, eager, and Wuffa recoiled from his intensity. He said in Latin, 'I've been expecting you.'

As evening fell the comet, suspended in dark northern skies, was brighter and more startling than ever.

While the novices slept in a stable in the Anglish village, the four guests were to stay the night in the fort, Ambrosias insisted. He prepared a meal. 'Eat, drink,' he said. 'A Roman is nothing if he is not hospitable.' He shuffled around with a plate of cut meat and a pitcher of ale. 'Of course I am grateful to my new Anglish neighbours down the hill, but I wish they could lay their hands on some good continental wine rather than this filthy German ale. Do you know, I tried to grow some vines here at one time, up against the southern wall of fort. Withered and died, the first hard winter. Ah, well!…'

Ambrosias's four guests, Ammanius and Sulpicia, Ulf and Wuffa, reclined on couches. This was the Roman way to take your meals, lying down. They were in a room carved out of the ruins of the old fort's principia, its headquarters building. It was a little island of Rome, with mosaics on the floor, frescos, crockery and cutlery, amphorae leaning against the walls of a minuscule kitchen. The floor was heaped with scrolls and wood-leaf blocks, the walls crowded with cupboards. The principia's original roof was long gone, but this one section had been roofed over by mouldering thatch.

Everything was worn and old, the pottery patched, the cutlery sharpened so often the knife blades were thin as autumn leaves, and the room was a mouth of dust and soot.

Ammanius quickly turned to the subject of Isolde. 'Do you know of her? If she ever existed-'

'Oh, she existed,' said Ambrosias. 'And I'm the living proof!'

'You?'

'I am a descendant of Isolde,' Ambrosias said. 'And therefore of Nennius, her father. I am the grandson of the grandson of the son of Isolde, in fact. And since she was born in Rome, as was her father, then I am a Roman, by descent.' He winked at Wuffa. "'The Last Roman." That's what you Angles say of me, isn't it?'

It would do Wuffa no good to point out the difference between Angles and Saxons, so he kept his silence.

Ammanius prompted, 'And the story of Isolde?'

It had happened nearly two centuries ago, Ambrosias said, in this very fortress. Isolde, then a young girl heavily pregnant, had been hauled all the way here from Rome by her own father, for purposes of his own. Far from home, Isolde had given birth, to the first of a line of five males that would eventually lead to Ambrosias himself.

And as she was in the pains of labour, she began to speak: to gabble in a tongue that was alien to herself and her father.

Ammanius was tentatively interested. 'She spoke in tongues, then. It is a common miracle. Did she speak of the Christ?'

'Oh, she mentioned Him,' Ambrosias said. 'But what was miraculous about it was that the tongue she spoke was German.'

Wuffa could see that that detail jarred with Ammanius's notion of what constituted a proper Christian miracle. But it intrigued Wuffa, for to him it made it seem more likely that something remarkable had happened, that this hadn't been a mere plague fever. What possible insanity could cause a Latin-speaking woman suddenly to spout German?

Sulpicia asked, 'And did she speak of the future? Was it really a prophecy?'

'Oh, yes, Nennius and the others with her recognised it as such immediately. They wrote it down, and it has been preserved by my family, in this place, ever since.'

Ammanius pressed, 'What did she say?'

Ambrosias sighed and gulped down a little more Anglish ale. 'Well, I'll tell you. Tomorrow we will discuss the past and the future and similar nonsense. But for now let us talk of other things. I am starved of educated conversation, stranded here among illiterate Germans. You are tired – or if you aren't, I am – and most of us are a little drunk on this scummy ale, I suspect.' He eyed Ammanius when he said this, and the bishop glared back.