Hitchens traced a large area on the map that seemed to cover Hyde Park and a hefty chunk of the West End.

‘The area just north of Marble Arch probably saw most of the fifty thousand executions. But all roads in this area lead to Tyburn. Until the eighteenth century, Park Lane was called Tyburn Lane and Oxford Street was called Tyburn Road.’

‘So they could dump a body anywhere from Hyde Park Corner to Oxford Street and still call it Tyburn?’ I said.

Professor Hitchens shrugged. ‘Theoretically,’ he said.

‘You really think they’d come back here with every plain clothes copper in the Met looking for them?’ Edie asked me.

I looked at Dr Joe for the answer.

‘I think they’ll find the temptation to stick to their ritual almost impossible to resist,’ he said. ‘If they kill again.’

‘And you think they will, Dr Joe?’

‘I don’t think they can stop themselves. I think they have set themselves the task of punishing all the wicked in the world.’

‘And who are they, Dr Joe? What kind of men are we after?’

‘They are men who clearly believe that justice has been thwarted. They’re obsessive about the judicial process – or at least their version of the judicial process. The Albert Pierrepoint obsession, the kangaroo court, the reading of charges. I would guess they have some experience of the law and they didn’t much like it.’

I waited for him to say what we were both thinking. But he just finished his frozen yogurt, because it would have sounded like blasphemy in here. So I said it for him.

‘They could be cops,’ I said.

‘Possible,’ said Dr Joe.

I turned to the woman with the swinging hair. She had her back to me.

‘How we doing on the voice recognition, Tara?’

She ignored me, and stood up to pitch an empty coffee cup into a wastepaper bin. She was a tall woman in flat shoes. One of those tall women who are never really comfortable with their height. Leggy. Slim. At first I had thought she was shy or self-contained. Now she just seemed indifferent to the people in this office. Especially me. She sat down and played the same piece of tape.

‘Do you know why you have been brought to this place of execution?’

I felt a surge of irritation.

‘Max?’ Edie said. ‘I don’t think that you were ever properly introduced. Tara is—’

And Tara Jones turned and looked up at me, as if seeing me for the first time as she pushed back her swinging black hair that fell across her pale and serious fabulous face.

And I finally realised that Tara Jones was deaf.