‘Three,’ he said. ‘James Sutcliffe killed himself in Italy.’

‘Of course. My apologies. Three of you.’

Then he looked at me. ‘I want to put an end to this,’ he said. ‘I want to help you in any way I can. I want to assist you in apprehending this murderer.’ He gripped my hand and he held my eyes.

‘I know,’ I said. ‘I know you do.’

We walked back inside and said goodbye.

As he crossed the central lobby, people approached him – lobby journalists, other MPs, women and men, all of them with the half-smile of the truly smitten – and he had a word for each of them. But he kept walking, he never broke that long easy stride, eventually disappearing into the northern corridor that leads to the House of Commons; and for the first time I appreciated his true power, and how much he had to lose.

It’s a five-minute walk from the Palace of Westminster to New Scotland Yard, and even though I did not have an appointment in Room 101, I thought it would be all right. They don’t get many visitors in the Black Museum.

‘I saw your film,’ said Sergeant John Caine, the keeper of the gate. He looked at me with his hard eyes, but there was no mockery in them. ‘Everybody saw your film, didn’t they?’

‘Will you watch it with me again?’

He was startled. ‘Why?’

‘It only lasts a minute. I want to ask your advice about something. I know you’ve seen it already, but will you watch it with me?’

There was an old computer on his desk. He pressed some keys and it wheezed into life.

‘My one might be quicker,’ I said.

He snorted. ‘All mod cons at West End Central, eh?’

I took out my Mac and we waited in silence as it powered up. We remained silent as I went to the social network site and found the profile page of Bob the Butcher. I began scrolling down all his postings about killing pigs and destroying worlds.

‘This one,’ I said.

I clicked on the link, and there it was, the mirth with no mercy of ‘The Laughing Policeman’ as the camera watched me crawl. Just as the pig appeared, I hit pause.

‘You see that?’ I asked Sergeant Caine. ‘By my right hand?’

He leaned closer. ‘A white line.’

‘A straight white line painted on grass,’ I said. ‘It’s a touchline. It’s the edge of a rugby field. The rugby field that’s closest to the trees at Potter’s Field.’

We stared at the image together. Then he shrugged.

‘So what?’ he said.

‘Doesn’t it prove that Bob the Butcher isn’t the killer?’

‘Why does it prove that?’

‘That film wasn’t shot when I got a good hiding. Whatever Bob tells his devoted followers, he had nothing to do with it.’

Sergeant Caine had a think.