I shook my head. ‘Not good enough, sir,’ I said. ‘And watch your mouth when you’re addressing my colleague. We have reason to believe that a serious crime was committed during the period when you were a student at Potter’s Field. I could take you in now if I wanted to.’

He laughed. ‘Really? And by this time next year you would be waiting outside in your new security guard’s uniform, watching over my children as they unload their mountain bikes from my wife’s Porsche Cayenne. Wouldn’t you?’ He paused, took a breath. ‘Tomorrow morning,’ he said. ‘Afternoon at the latest. I promise. I want it to be over.’

‘It will be,’ I assured him. ‘We can help you. But you have to help us: what happened at that school?’

‘What happened? I’ll tell you what happened. They break you down and then they build you up. That’s what they do at those fine old English schools. That’s what your parents are paying them for. They take you apart bit by bit and they put you together again in their image. They take scared little boys and they turn them into captains of industry, leaders of the land, future Prime Ministers.’

He took a long pull on his cigarette.

‘The first day I met Peregrine Waugh – I was thirteen years old, he was just an English master – he drew a single chalk mark on the wall just above the blackboard. “That is Shakespeare,” he said. And then he drew another chalk mark, at the very top of the blackboard. “That is T. E. Lawrence.” And then he got down on his knees – we were all laughing madly, of course – and drew a third chalk mark just above the floorboards. “And that is you.”’ Salman Khan smiled a crooked smile as he waved his cigarette. ‘And we took it from there.’

It was a busy morning at the Black Museum. A dozen young cadets in uniform were squeezed into Room 101. Sergeant John Caine contemplated them without pleasure or pity.

‘Ground rules,’ he said. ‘You do not touch anything. You do not photograph anything. You do not take a souvenir that you think nobody will miss. Believe me, I will miss it. Everything here is far older and far more valuable than you are.’

A few guffaws. But Sergeant Caine wasn’t laughing.

‘So show respect,’ he continued. ‘And keep your sticky paws in your pockets at all times.’

He unlocked the door to the Black Museum.

‘Go on. Off you go.’

They set off, excited and laughing, like big kids on a school trip. When the last of them had shuffled into the first room, the one mocked up to look like the original museum in Whitehall, Sergeant Caine turned his attention to me.