It was night in the picture. A long shot of some kind of party in a large white tent. Waiters hovered with trays of champagne flutes. Parents and boys and masters and guests stood around grinning and swilling. Another Potter’s Fifth had gone well. And on the far side of the party I could see a boy and a girl half turning away from the camera with a smile.

‘It’s Ben King,’ I said.

‘It’s Ned King,’ Wren said. ‘Look closer. You can just about see the scars on his face where his brother glassed him over the Coco Pops.’

‘And the girl?’

She was young, blonde, pretty. A ponytail bobbing, the shadow of a secret smile as she turned away. She was in T-shirt and jeans, far too casual for an event like the one in the big white tent, far too casual for Potter’s Fifth.

Wren smiled and shook her head. ‘I don’t know. It could be just a girl. You talked to Ned King, right? You and Mallory.’

I remembered the night out at Brize Norton when Captain King and his regiment were about to get on the Hercules for Afghanistan.

‘The killings had only just begun,’ I said. ‘Ned King was leaving. Mallory was gentle with him. Impressed by him.’ I stared through the magnifying glass at the image of the boy and girl at a party long ago. ‘We both were.’

‘Well, someone has to talk to Captain King again,’ Wren said. ‘When he gets back.’

‘Monty,’ I said, ‘would that have been your only reason to go to the school? Potter’s Fifth?’

He nodded and eased himself out of the chair. ‘Apart from pictures we needed for the files,’ he said. ‘When there was a new Head Master, they would always want a shot of him. And the statue of Henry. The mainstay of our tourist industry.’ He was bent over the coffee table, not risking getting down on his knees. ‘And the grave, of course. The grave of the royal dogs.’ He tapped a ragged cardboard file. ‘There you go.’

These were not contact sheets. These were glossy eight-by-ten shots. Some of them were of the Head Masters of Potter’s Field, formal portraits. Peregrine Waugh was the most recent but there were also men long retired, long dead. And there were close-up shots of the grave of the royal dogs, taken in all kinds of weather, and all kinds of seasons.

I held one in each hand. One was black and white, the stone slick with rain, the grass around it untrimmed. The other was in colour, the stone reflecting a white bar of summer sunshine, the grass border neat and cut.

But that wasn’t the only difference between the two photographs.

‘Look at the inscription,’ I said. ‘Look at the epitaph on the grave.’ I held up the black-and-white shot. ‘The words are missing on this one.’