Carrying the triple espresso I had bought at Bar Italia in Soho, I went up to the Major Incident Room, MIR-1, on the top floor. MIR-1 would be the centre of Homicide’s murder investigation. It was a large suite of connecting rooms with a computer station at every desk and it was now completely empty apart from DCI Mallory, who stood cradling a carton of takeaway tea as he stared at a blank whiteboard.

‘You’re early,’ he said. ‘Morning briefing’s not for an hour.’

‘I thought I’d be the first to arrive, sir,’ I said. ‘Look keen and all that.’

He laughed. ‘I like to spend a bit of time figuring before I start opening my mouth,’ he said. ‘What could possibly connect a wealthy investment banker and a homeless heroin addict?’ He shook his head. ‘I don’t know. I have no idea. And I need to know. At least I need to have a theory.’ He sipped his tea. ‘Have you heard of the Golden Hour principle?’

I nodded. ‘It means early action can secure material that would otherwise be lost. Witnesses remember things more clearly. Offenders can still be nearby. CCTV footage hasn’t been deleted. The longer we leave it, the harder it all gets.’

‘I believe in the Golden Hour principle as much as the next man,’ Mallory said. ‘But I also believe in what the old SIOs call “creating slow time”. Meaning you need to put your foot on the ball and have a figure; meaning you have to leave time for figuring as well as action.’

He was such a soft-spoken man that it took me a moment to realise that by coming in early I had intruded upon his private time – his figuring time. He must have seen the alarm on my face.

‘Why don’t you go down to the basement?’ he suggested kindly. ‘See if you can find our weapon.’ He held out an A4 file. ‘Take this with you.’

‘Sir.’

I bolted my coffee and went down to the basement. The lift doors opened on to a low-ceilinged room where row upon row of canteen dining tables were covered with knives.

A young uniformed officer was filming them and making notes on a clipboard. He looked like a tourist in some exotic marketplace.

‘Help you, sir?’ he said.

‘I’m looking for a knife,’ I said.

‘What kind of knife, sir?’

‘A knife that can do this.’

There were four photographs inside the file. Two of Hugo Buck’s corpse and the other two of the homeless man. They were all graphic close-ups of the fatal wounds. I held them up for the constable and saw the blood drain from his face.

‘Go ahead, sir,’ he said. ‘We’ve got all sorts here.’