Dyer suddenly saw Gilles peering up from the depths, fanged. Perhaps it was his name. The only place to grab a pike without getting your fingers severed off was under the gill plate into the lower jaw.

‘Oh, I think he has more in common with Pierre. He’s a great fan of your son. He says Pierre could easily be goalie for the Oxford Schools team.’

But Gilles wasn’t to be caught like that. He said with sinister emphasis: ‘I have been wondering if perhaps, Jean, you know more than I know.’

‘About Oxford Schools?’

‘About Rustum Marvar.’

The only sound in the room, the portentous jingle of his keys.

‘Perhaps I do.’

Gilles sat back, hands quiet, waiting to hear what Dyer had to say.

Dyer raised his eyes. ‘I did in fact come into possession of a Ziploc sandwich bag of Marvar’s – but only for a short time. I lost it, unfortunately.’

‘What was in it?’

‘A post-it note.’

He waited for his answer to sink in. It sounded so silly. But Gilles did not think so.

‘Just one?’ His voice was shrill with a businessman’s impatience.

‘That’s right.’ Dyer saw no need to mention the two extra blank notes. ‘With some kind of mathematical formula written on it.’

‘And before you lost it, you made a copy?’

‘I didn’t.’

‘You didn’t have time?’

‘Not that. It’s hard to explain, but I felt it wasn’t something Marvar would have wanted me to do. He told me he had very deliberately made one copy only, and this, I presumed, was it.’

‘Did you believe him?’

Dyer chose his words with care, like biting off the right length of nylon for a fly. ‘I believe that he believed he had achieved something remarkable.’

Gilles was looking at him as Katya had. ‘No, do you believe he only made one copy?’

Dyer intuited that an awful amount depended upon his answer. Not only that, but none of his answers so far had come as a surprise to Gilles. The feeling overwhelmed him that Gilles was involved with the second break-in, and he had read the substitute post-it note inspired by Basil Bunting’s poem. Or if he hadn’t read it, then he was in contact with others who had, and was coming to the conclusion that it was the ravings of a madman. But first he needed to be convinced beyond doubt.

‘Yes, I do believe him,’ Dyer said.

‘This post-it note, can you remember what was written on it?’

‘God, no! It was an immensely long algorithm – ten, eleven lines of figures, covering both sides.’

‘Just figures?’

‘Plus some letters. Oh, and a quote from a German scientist.’

Gilles was not interested in Otto Hahn. ‘Could Marvar have remembered the algorithm by heart?’