I stared at him. I didn’t believe in ghosts, or fairies or gods, and for the last couple of days I’d been like a man watching a magic show — I’d expected a magician to step out from behind the curtain and ask me to pick a card, any card. I wasn’t ready to believe in ghosts, but that’s the thing about empirical experience — it’s the real thing.

And if ghosts were real?

‘Is this where you tell me that there’s a secret branch of the Met whose task it is to tackle ghosts, ghouls, faeries, demons, witches and warlocks, elves and goblins … ?’ I said. ‘You can stop me before I run out of supernatural creatures.’

‘You haven’t even scratched the surface,’ said Nightingale.

‘Aliens?’ I had to ask.

‘Not yet.’

‘And the secret branch of the Met?’

‘Just me, I’m afraid,’ he said.

‘And you want me to what … join?’

‘Help,’ said Nightingale, ‘with this inquiry.’

‘You think there’s something supernatural about the murder?’ I asked.

‘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.