A security guard emerged as we were coming through the gardens. He stared at us with suspicion until a woman’s voice called out to him: ‘It’s all right, Charlie, they’re with me.’

The security guard stepped back, and there was a young woman in a wheelchair. She smiled when she saw Mallory.

‘Sir,’ she said, and her smile grew bigger as she saluted.

‘Hello, Carol,’ he said, leaning forward to kiss her on each cheek.

He introduced me and we waited while she turned her wheelchair in a tight space. I went to help but she stopped me with a curt, ‘I’ve got it’.

We followed her down a narrow corridor that led to the main hall. The lights were off. In the darkness Spitfires and Messerschmitts and Hurricanes and V2 flying bombs hung above us, frozen in mid-flight, frozen in time.

‘How can I help you, sir?’ Carol said.

Mallory was opening his briefcase. ‘We’re looking for a murder weapon, Carol.’ He handed her the file.

She slowly leafed through it.

‘Our conjecture is that the weapon was designed for cutting a man’s throat,’ Mallory said.

‘Your conjecture is right, sir,’ she said, smiling again. ‘I know just the thing. We should have one in storage. Please follow me.’

The knife was just under twelve inches long, most of it blade, a blade that was long and thin and double-edged, designed to slip easily into human flesh and then cut through it without fuss. Mallory handed it to me, and I was surprised to find that it weighed next to nothing. The handle had a comforting ring grip, sitting easy in my closed right fist, and the thing conveyed a sense of terrible power.

‘The Fairbairn-Sykes fighting knife,’ Carol said. ‘Developed by William Ewart Fairbairn and Eric Anthony Sykes when they were policing in Shanghai before the war.’

‘The commando knife,’ Mallory said. ‘Of course.’

I could not get used to how light it felt in my hand. It was so easy to hold. A sharp stabbing point, a good cutting edge. The double-edged blade glinted in the half-light of the storage room.

‘This was invented by two policemen?’ I said.

Carol nodded. ‘W. E. Fairbairn was an expert in close-combat fighting. This was designed to be the perfect fighting knife but it turned out to be the perfect killing knife. Standard-issue sidearm for all World War Two commandos. If the Nazis caught you with one of these, you were shot on the spot as a spy. Most knives are not specifically designed to kill – apart from the Fairbairn-Sykes. It looks easy to use, but it’s not. You have to know which arteries are closest to the surface and unprotected by clothing. Give it here, will you?’