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.’