‘The usual way. I passed my National Investigations Exam. I’m working towards my PIP Level 2 Portfolio. You know – demonstrating competence, all of that?’

‘What you doing here?’

‘I’m the bagman on the double homicide.’

‘Our bagman? The bagman’s usually a detective sergeant or detective constable.’

‘Cuts,’ she said.

The bagman was the lowest rank of the investigation, answering directly to the senior investigating officer, DCI Mallory, and giving assistance wherever it was needed. I remembered how calm Wren had been on the day she and Greene found Buck’s body and I had no doubt she could do the job.

‘So this is where it all happens,’ she said.

‘This is where some of it happens,’ I said. ‘There’s plenty of legwork outside. I’ve learned that murder is labour intensive. In the Yorkshire Ripper case, the law checked over five million car registration numbers.’

‘Yes, and they still cocked it up,’ she said. ‘Did you see the new Bob the Butcher post?’

She stabbed a few buttons on her laptop. The social network site came up. It was nothing I hadn’t seen before. The later photograph of Robert Oppenheimer. The same apocalyptic boasting: ‘I am become Death, the destroyer of worlds. Kill the protectors of the rich. Kill all pigs.’

‘Seen it,’ I said.

‘But did you see the link?’ Wren said. ‘That’s new. And you’re in it.’

She clicked on the link. And there I was on my hands and knees, a thin trickle of blood running down the side of my face from a wound in my temple. Another sliding down the side of my neck. My mouth was open and my eyes were fixed on nothing.

And I was crawling.

There was only a few seconds of footage, but it kept repeating, and looping back on itself, so it looked like I was crawling forward, and then reversing, as if doing some comic jig. In one corner of the screen the pig edged towards me, its eyes bulging with terror. Our faces would almost touch and then we would back away from each other. And then the mad dance would begin again.

There was music, I realised. Not really music at all. Just mad laughter, on and on, hysterical screams of laughter over an oompah-oompah beat as the pig and I crawled towards each other and then backed away. And did it again. Laughter in the dark, laughter from the grave.

‘Who filmed you out there?’ Wren said.

‘I don’t know.’

‘Did he film you, the killer? Did Bob make this film?’

‘I doubt it,’ I said. ‘Not his style.’

‘I would have thought it was exactly his style,’ Wren said. ‘I Shazamed it. That song. It’s some old music-hall number. “The Laughing Policeman” by Charles Jolly.’

As if on cue, Charles Jolly stopped laughing and started singing.

He said, ‘I must arrest you.’

He didn’t know what for.

And then he started laughing