‘The big problem is that, after leaving the kill site, you can’t tell us if you were walking in a circle or a straight line,’ Hitchens said. ‘So we don’t know if the station where they found you is next door or several miles away.’ He looked at Whitestone. ‘I can’t overemphasise how vast London is below ground. All those forgotten tunnels, all those uncharted hallways, all those derelict passages – there is a parallel London with layer upon layer of geologic time.’

‘We appreciate all your help, Professor,’ said Whitestone.

‘Come on, Hitch,’ I said, indicating the frozen image of the kill site. ‘Look at that room! Those tiles have to be at least a century old. Doesn’t it ring any bells?’

‘I wish I could be of more help,’ said Professor Hitchens, touching his mouth with those nicotine-stained fingers. I could feel the lopsided welt around my neck throbbing with my frustration. Then I rubbed his great bald egghead to comfort him.

‘No problem, Hitch,’ I said.

You’re not the only history man in town, I thought.

*  *  *

The Black Museum is cold and dark.

The temperature is low in order to preserve the microscopic particles of human flesh that still attach themselves to certain exhibits – some of them 140 years old – while the restrained lighting prevents the Museum’s exhibits from fading. The subdued lighting does something else. It gives London’s most secret room an aura of quiet menace.

When I arrived at Room 101, New Scotland Yard, Sergeant John Caine was just finishing a tour for a dozen uniformed police cadets from Hendon.

‘Give me give a couple of minutes, Max.’

I stood at the back of the tour group.

The young men and women were in a sombre mood. The Met calls the Black Museum – or the Crime Museum, the official name that is increasingly used – a learning resource. And it will certainly tell you more than you really want to know about human nature.

The fledgling cops were staring up at a high shelf on which was displayed a collection of death masks, three-dimensional plaster casts of heads of executed men and women. They were all dark brown, the shade of a burned coffee bean, apart from the very oldest, which had turned jet black over time. The heads were all smooth and their eyes were all closed. But there the similarities ended – the death masks had been taken from faces young and old, fat and lean, male and female.

‘Are they real?’ said a young copper who looked as though he only shaved once a week.

‘All of them are real but none of them will bite you,’ said John Caine, standing behind the group. ‘They were all taken from the deceased immediately after they had been executed for murder.’

John cleared his throat and the cadets tore their eyes away from the macabre masks to listen to his closing speech.