‘To catch a grayling, you use a small fly and bend it out.’ Dyer heard his father’s tobacco-scented voice in his as he explained to Leandro how grayling had their own way of feeding. ‘They come up from the river bed and seize the fly in a different motion to trout, who attack it parallel and turn their head.’ He handed the rod to his son.

Leandro made a number of false casts before letting the line out with a splash. He snagged his next cast in weeds. Dyer waded out to retrieve it. ‘Don’t despair, be patient, you can’t learn it all in a day,’ and reminded Leandro of the tangles he’d got into on the rugby pitch.

‘Try over there at forty-five degrees. Remember to strip in fast – and strike as soon as you feel something.’

Hours passed. By the time the rain had stopped, Leandro was casting better. He was fairly positive that he had felt two tugs.

They ate a late lunch on the bank, sandwiches provided by the hotel. It was already mid-afternoon, a breeze was stirring the tall damp grass. A flock of doves settled on the top of a tree and flew off, flapping like pages in the wind.

Lunch over, Leandro leaped to his feet – impatient to try out his carbon rod. Dyer set this up. Then, after watching his son, he left him by the stepping stones and walked upstream with the split cane, to a long beat in shadow under the trees, just far enough away to still have Leandro in sight. He wanted to be on his own for a while, take stock.

After a morning on the river, Dyer no longer had the insidious sense of being watched which had oppressed him continually since Rustum Marvar stayed the night. Leandro, too, seemed calmer. Mr Tanner had acted fast in letting him choose a new password to access the school’s computer system; on Dyer’s advice, Leandro took a line from “The Prelude”, and the menacing emails had immediately stopped.

Away from Oxford, Dyer was able to focus with greater clarity – in a manner not possible before – and think about what to do with Marvar’s post-it note.

Three more days, was the deadline he had set himself. The library book was reserved until the following Tuesday, and then Dyer needed to make a decision. Did he leave the algorithm folded inside the last chapter for an unknown researcher to find – and inevitably throw away? Did he choose a new book to slip Marvar’s information into, and then another, until he ran out of books? Or did he find some other person and press it into their palm?