‘Work out from the photograph applying the ABC principle at all times,’ DCI Mallory warned us, moving between the workstations where we hunted, our phones on permanent charge and HOLMES2 – the Home Office Large Major Enquiry System, the mainframe police computer – flickering on the screens in front of us. ‘Assume nothing. Believe nothing. Challenge and check everything.’

The sky was flat and white above the rooftops of Mayfair when Mallory faced us for his afternoon briefing. He ran a hand across his scalp and took a joyless gulp of cold takeaway tea.

‘Right,’ he said. ‘What are we looking at?’

We were looking at the large flat plasma screen on the wall of MIR-1, and on the screen was the photograph that had been in the home of Adam Jones’s mother, and also on Hugo Buck’s desk.

DI Whitestone, the MIT’s deputy SIO, cleared her throat and said, ‘Sir, this photograph was taken at Potter’s Field College in the spring of 1988. Potter’s Field College is a boarding school for boys aged thirteen to eighteen on the border of Berkshire and Buckinghamshire. The seven boys in the photograph are all wearing the uniform of the school’s Combined Cadet Force. On the far left of the picture is Adam Jones. And on the far right is Hugo Buck.’

Next to the screen was Mallory’s wall – a massive whiteboard with the physical photograph of the seven smiling boys, eight inches by ten inches out of its frame, looking tiny tacked up there, and next to it a selection of pictures of Jones and Buck taken at the scenes of their murder and again at their autopsy at the Iain West.

‘The Combined Cadet Force is the Potter’s Field version of the Officer Training Corps,’ Whitestone continued, ‘which they still go in for at a lot of these old public schools.’

‘And Potter’s Field is one of the oldest,’ Mallory said. ‘Five hundred years old? It has to be as old as Harrow, St Paul’s, Westminster, Winchester, Rugby and Marlborough. And of course Slough Comprehensive.’ He glanced at our blank faces. ‘Eton,’ he explained. ‘They call themselves Slough Comprehensive. It’s their little joke. What’s our source for the photograph?’

I said, ‘Sir, this is the copy I obtained from the mother of Adam Jones.’

‘And we’re absolutely certain it’s the same one that was on the desk of Hugo Buck?’

DI Gane looked up from his laptop. ‘The one at the bank had some superficial damage, sir. There was a blood trace that had seeped under the glass. But it’s the same photograph all right.’

‘And were Jones and Buck in contact after leaving school?’ Mallory asked.

‘Looks like they went their separate ways, sir,’ I replied. ‘Jones’s mother hadn’t seen Hugo Buck in nearly twenty years. She wasn’t aware of his death.’

Gane said, ‘Doesn’t she read the papers?’