The pathology report on the weapon was more interesting. Its shape matched the fatal wound track in James Gallagher’s back, a shallower wound in his shoulder, and was a probable match for three cuts to his left and right hands – probably defensive wounds. The blood covering the weapon was a DNA match for James Gallagher and splatter analysis indicated that he might have pulled the weapon out of himself while lying on the tracks. Lovely. However, there were traces of a second blood type on the edges near the ‘handle’ which should be amenable to Low Copy Number DNA testing, the downside being that that the results would take at least until January to come through. An attached note from Seawoll told us to check for hand injuries when taking statements. It takes more force than you think to stab someone to death and the human body is full of pesky obstructions – like ribs for example. Inexperienced knife fighters frequently cut themselves on their own blades when the momentum of their thrust drives their hand down the knife. That’s what a cross guard on a combat knife is there to stop and what makes it relatively easy to catch knife murderers – look for the wounds, match the DNA, it’s a fair cop guv, hello Pentonville. That’s the thing about hard evidence. It’s difficult to wriggle out of in court. No wonder Seawoll and Stephanopoulos weren’t hassling me. They probably figured it was just a matter of time before they swabbed the inside of the right mouth.

Assuming the DNA turned out to be human.

The mud on James Gallagher’s boots turned out to be an appetizing mixture of human faeces, shredded toilet paper and a combination of chemicals that placed him in a working sewer within eight hours of his death. I dug out Sergeant Kumar’s number and was routed through to his airwave. I heard crowds and a PA system in the background. He was definitely on duty. I told him about the sewer mud on the boots.

‘We’ve already been asked about that,’ said Kumar. ‘There’s a gravity sewer that runs below Baker Street and at the end there’s another which runs below Portland Place. But there’s no direct access anywhere on the stretch of track between the two. You walked it with me – there was no way for him to get onto that section.’

‘What about a secret passage?’ I asked. ‘I thought the Underground was full of them.’

‘Secret from members of the public, yes,’ said Kumar. ‘Secret from us – no.’

‘You sure about that?’ I asked and Kumar made a rude noise.

‘I did find some interesting CCTV footage from last Sunday,’ he said. ‘Very irresponsible behaviour by a man and a woman and what looked suspiciously like a child in an enormous hat. On tracks near Tufnell Park – ringing any bells?’

‘Really,’ I said. ‘Were they easy to identify?’