Before he switched off his computer, Cooper checked the Matlock Bath webcam to see if it was still running, as the site claimed.

When the picture came up, he saw it was already dark in Matlock Bath. He looked at his watch. Six o’clock. He hadn’t realized it had got so late.

When he looked back at the screen, the webcam picture was reloading. Now the headlights of two cars were approaching the camera. But the only other colour in the image came from strings of lights hung along both banks of the river, and across the road. Some of the illuminations came close to the camera, mounted on the roof of Life in a Lens. Coloured lights also framed the iron girders of the Jubilee Bridge. The bridge was reflected on the surface water as a black, shapeless mass that disappeared into the trees on the other side of the river.

Across the river and into the trees. Where did that phrase come from? It must be a song, or possibly a book title. It made him think about the gunman who’d shot down a harmless middle-aged woman. If he was a professional, then there was something that no one was admitting out loud. He would be long gone from the area by now.

Never mind across the river and into the trees – their suspect could have been on the other side of the world before the clock even began to tick on the Shepherd enquiry.

Later that evening, Liz Petty sat in the upstairs room of Aitch’s Wine Bar in Bakewell and accepted a glass of Merlot.

‘Thanks, Ben.’

‘Cheers.’

Cooper sat down next to her with his bottle of beer. The remains of their dinner had been cleared away, and he was starting to wonder whether chocolate truffle cake would go all right with the plum-and-chilli sauce that had been on the char-grilled chicken.

‘Anyway, Quinton Downie was right,’ said Petty. ‘Fire can be one of the most difficult things to investigate. So many factors influence its behaviour that a scene can be very misleading.’

She took a drink of wine and gazed out of the window at Buxton Road. Liz lived just off Fly Hill, a couple of minutes’ walk from the wine bar, in a three-storey terraced cottage she rented from her uncle. The third-floor bedroom had a terrific view beyond Bakewell towards the golf club.

‘I remember something we were told on the course I did,’ said Petty. ‘It was a real incident, with photographs. A young child who’d died in a fire. It made me think of that case when I heard there were two children involved at Darwin Street. I know these two weren’t burned to death, but still . . .’