‘Why don’t you tell me what your witness had to say,’ he said, ‘and then we’ll see where it goes.’

So I told him about Nicholas and the change of clothes by the murdering gent. About the CCTV coverage and the Murder Team thinking it was two separate people. When I’d finished, he signalled the waitress for the bill.

‘I wish I’d known this yesterday,’ he said. ‘But we still might be able to pick up a trace.’

‘A trace of what, sir?’ I asked.

‘The uncanny,’ said Nightingale. ‘It always leaves a trace.’

Nightingale’s motor was a Jag, a genuine Mark 2 with the 3.8 litre XK6 engine. My dad would have sold his trumpet for a chance to own a car like that, and that was back in the 1960s when that still meant something. It wasn’t pristine: there were some dings on the body work and a nasty scratch on the driver’s side door, and the leather on the seats was beginning to crack, but when Nightingale turned the key in the ignition and the inline-6 rumbled, it was perfect where it counted.

‘You took sciences at A level,’ said Nightingale as we pulled out. ‘Why didn’t you take a science degree?’

‘I got distracted, sir,’ I said. ‘My grades were low and I couldn’t get on the course that I wanted.’

‘Really? What was the distraction?’ he asked. ‘Music, perhaps? Did you start a band?’

‘No sir,’ I said. ‘Nothing that interesting.’

We headed down through Trafalgar Square and took advantage of the discreet Metropolitan Police flash on the windscreen to cut through the Mall, past Bucking-ham Palace and into Victoria. I knew there were only two places we might be going; Belgravia nick, where the Murder Team had their incident room, or Westminster Mortuary where the body was stashed. I hoped it was the incident room, but of course it was the mortuary.

‘But you understand the scientific method, though?’ asked Nightingale.

‘Yes sir,’ I said, and thought, Bacon, Descartes and Newton – check. Observation, hypothesis, experiment and something else that I could look up when I got back to my laptop.

‘Good,’ said Nightingale. ‘Because I need someone with some objectivity.’

Definitely the morgue then, I thought.

Its official name is the Iain West Forensic Suite, and it represents the Home Office’s best attempt to make one of its mortuaries look as cool as the ones in American TV shows. In order to keep filthy policemen from contaminating any trace evidence on the body, there was a special viewing area with live autopsies piped in by closed-circuit television. This had the effect of reducing even the most grisly post-mortem to nothing more than a gruesome TV documentary. I was all for that but Nightingale, on the other hand, said that we needed to get close to the corpse.

‘Why?’ I asked.

‘Because there are other senses than sight,’ said Nightingale.

‘Are we talking ESP here?’