For one thing Isolde, nineteen years old, didn’t know anybody in Rome who had even been as far as Gaul, much of which was in the hands of foederati, German ‘allies’ of the empire. All Isolde’s friends knew about Britain was that giants had built a mighty Wall across the neck of the island to keep out capering monsters.

Nonsense, said Nennius, her father, predictably. You could tell when he got really angry because a pink flush spread all the way up his round cheeks to the shaven patch at the top of his head. Britain was just a place, its inhabitants just people, not monsters - and there was a Wall, but it had been built by Romans, not giants. Why, it was less than a decade since the British Revolution, when ‘ragged-arsed rebels’ had refused to pay their taxes. Britain had been detached from the empire many times before, and would no doubt be rejoined to the mother state when time and resources permitted.

‘And anyhow,’ he told her with a certain malicious glee, ‘we’re off to visit a cousin of mine, who lives on the famous Wall. We share a grandfather, cousin Tarcho and I, a slave who became a soldier called Audax, who was at the heart of the Prophecy story. And do you know how I happen to have a cousin there? Because you and I are British ourselves, daughter - a couple of generations removed, but British all the same ...’

Nennius’s latest scheme was all to do with a Prophecy, he said, a Prophecy lost and now partially found again, a Prophecy made but never fulfilled - a Prophecy that might have shaped the world. The key to reconstructing this puzzle, he believed, and perhaps even to recovering the Prophecy itself, lay in Britain. And so because of this old man’s legend Isolde must travel beyond the empire itself.

Isolde had learned long ago that it did her no good to argue. Her whole life had been shaped by her father’s ambitions, and so it was now. But as they crossed a Gaul in which you heard nothing spoken but German, and as they took to the sea in the leather-sailed boat of a blond Saxon trader with bad teeth, she felt terribly vulnerable. She was a pregnant woman accompanied only by an absent-minded old man. Not only that, her stomach churned with every tip and rock of the boat. The trader offered her a remedy, a cold tea of German herbs, but Nennius forbade her even to try it.

She tried to tell herself she was safe with her father, but she had never believed that even as a small child. He simply didn’t pay enough attention to you to make you safe.