There’s a moment in the middle of a hangover when you have a sudden penetration and you see the world with complete clarity. You have it seldom, but when this flash comes it seems as if it’s calculated, like an oracle. Dyer had experienced it once or twice when he rowed. Instincts take charge and you see what is important, what not, and how to plunge ahead. The Buddhists probably had a word for it. Thinking too much was the problem. The answer came to him when he let his body and instincts do the reacting, with not too much brain.

Like a man smoothing out something he had crumpled, Dyer saw what he must do.

‘That can go back,’ he said to the librarian. He handed her the collection of first-hand accounts describing Cabral’s voyage to Brazil.

She took it without glancing up.

‘You don’t want to renew this?’

He looked down at her white head. ‘No.’

The way she diverted her gaze to hide that she’d been watching him. She walked as though she wore velvet slippers, her dark hair in a Suebian knot.

A sporty-looking woman he had not seen before descended in quick silent steps behind him. He reached the bottom of the staircase and pushed out into the street through the open glass door. Then something made Dyer turn and pull it shut.

In the ferment of knowing how he was going to dispose of Marvar’s algorithm, Dyer had let go the danger of carrying it on his person. But he had learned to trust his presentiments. After a moment of imbalance, he was wide awake.

At an unswerving pace he set off, as though heading in the direction of the Martyrs’ Memorial. Crossing the road at a diagonal, he waited for a taxi to go by, before cutting between it and a Waitrose van. As the van moved slowly forward, shielding him momentarily from the Taylorian, he ducked left behind it.

Yards away – about to pull out – was a red double-decker Citysightseeing bus. Dyer ran towards it, keeping his head low. He jumped on board as the doors hissed shut, and then thrust his way past seats occupied by smiling Chinese tourists, taking cover behind a broad-faced man who stood in the aisle holding a microphone.

Through the rear window, Dyer watched the woman hurry from the library. He shrank back, out of her line of vision. When he next risked a look, she had stopped on the pavement, and was searching up and down St Giles, peering intently at the van, the passing cars, the receding bus, before reaching into her tight-fitting anorak and, after a flicker of hesitation, jogging off in the opposite direction.

‘Well, hello, again,’ said the man, who wore a black blazer with brass buttons on it and a familiar striped tie. ‘Come to join my tour? Let me introduce you to this vibrant and cultured city.’