Hugo Buck on the right – dark and mannish, already looking like he needed to shave every day, confident in his looks and his strength and his place in the world.

I was looking harder at the photo, really seeing it for the first time. They were not all the same.

They were not even all smiling.

In the middle of the group was a serious, delicate-looking boy in dark glasses, his fringe swept back off a high forehead.

He was flanked by the twins – identical, I could now see. Tall, good-looking and cold. One of them had a jumble of jagged scars running down one side of his face.

Next to the twin with the scars, between him and Adam Jones, there was a chubby, leering adolescent, the tip of his tongue exploring a corner of his mouth.

And on the other side, next to Buck, there was a grinning dark-skinned youth, possibly Indian, far taller than the rest.

Only their uniforms were the same.

‘So what exactly is this Combined Cadet Force?’ Gane said. ‘Posh rich kids playing soldiers?’

‘They take it seriously,’ Mallory said. ‘A significant number of boys take up commissions in these schools. I bet the British Army is still the biggest employer of Old Potter’s Field Boys. Even now. They say that Old Etonians go into the army, politics and acting while Old Potters go into the army, banking and jail.’

I wondered how Mallory knew so much about it.

‘But what’s the point of them?’ Gane persisted. ‘The Officer Training Corps. The Combined Cadet Force. Why do a bunch of public schoolboys need to dress up as soldiers?’

Whitestone looked at her notes. ‘It’s been around for quite a while. The Potter’s Field Combined Cadet Force was founded in 1805 as the Potter’s Field Rifles.’

Mallory smiled with affection at Gane, his round John Lennon glasses glinting in the overhead lights. ‘That year ring any bells, DI Gane? No? The country thought that Napoleon was about to invade. He had an army of two hundred thousand men on the coast of France. These grand old schools were preparing a home guard. A youth movement to repel the invaders.’

‘And did Napoleon bottle it, sir?’ Gane asked. ‘Excuse my ignorance.’

‘Yes, Gane, Napoleon seriously bottled it.’ Mallory looked at the screen. ‘But whether they called them an Officer Training Corps or a Combined Cadet Force or something else, the public schools all kept their Rifles.’

‘They join at fifteen,’ Whitestone said, ‘around the age of the boys in the photograph. And they’re taught by regular serving army officers. Leadership skills, signals. And how to shoot. They have a range weekend where they use live ammunition.’

‘Any other business?’ Mallory said.

‘Bob the Butcher, sir,’ Gane said.

And then he turned on me.