I saw rags in glass cases, rusty medals in presentation cases, framed photographs of smiling men who had died screaming a lifetime ago.

‘Nice place,’ Wren said. ‘My old man would love it. Is that a real Luger P08?’

She was indicating a handgun in a dark stained-glass box.

‘Replica,’ he replied, looking at me. ‘It doesn’t fire.’

I opened my bag and removed a file. Inside it was a collection of eight-by-ten photographs. I spread them on top of a glass cabinet and felt a pulse of pain freeze my lower back. The photographs were all of a knife that was designed to cut a man’s throat. I leaned forward, my palms on the glass case, easing my tailbone and my breath out at the same time. It wasn’t a proper dog stretch, but it made me feel better.

‘I know you,’ Cage said. ‘Where was it?’ He clicked his fingers. ‘The Laughing Policeman film on YouTube.’

‘He’s dead famous,’ Wren said, flicking through a rack of ragged army jackets as if she was in Miss Selfridge. ‘He went viral.’

I looked at Cage. ‘You don’t know me, OK?’ I said.

‘OK,’ he said.

‘Do you know what these are?’

He looked at the photos one by one.

‘It’s a Fairbairn-Sykes commando dagger. They’re all Fairbairn-Sykes commando daggers. These are all different generations of the same weapon. I could be more specific if I could see the knives. This one with the green handle is marked PPCLI. That’s Canadian – Princess Patricia’s Canadian Light Infantry. This one with the French writing – LE COMMANDO INOX, see that? – was used by French Special Forces in Algeria between 1954 and 1962. Laymen think that the Fairbairn-Sykes was only used in World War Two. But Wilkinson’s kept producing them after the war and sold them to Special Forces all over the world. This is about Bob the Butcher, isn’t it?’

‘Did you ever sell one of these knives?’

He shook his head. ‘The only place I ever saw one was in a museum.’

‘The Imperial War Museum?’

‘Yes.’

‘What kind of clients do you get in here?’

He was watching Wren browsing the store.

‘Collectors,’ he said.

The sound of muffled laughter. Wren had put on a gas mask.

‘My dad would love this,’ she said. ‘Christmas is coming.’

‘It’s six hundred quid,’ Cage said. ‘You break it, you bought it.’

Wren removed the gas mask. ‘Maybe I’ll stick to socks and aftershave.’

‘What’s the appeal?’ I said.

Cage looked at me as if he couldn’t believe what he was hearing.