Cooper decided to drive down through Cromford to reach Matlock Bath. It was a relief to get out into the countryside again. This was his natural environment, not the stuffy meetings to discuss assassinations and organized crime, where he felt uneasy and out of his depth. Let Diane Fry have that side of the job, if she wanted it.

Emerging from the canopy of trees on the Via Gellia, he passed a little tufa cottage. This was one of the area’s most photographed buildings, and it looked just the way he remembered it – a house made of grey, spongy stone, with wisteria growing up the wall and geraniums in the window boxes, like something out of a fairy tale.

A few yards further on, the road swung to the right by the old Pig of Lead pub and the mills nestling in Bonsall Hollow, below Ball Eye Quarry. There was a quirky little bookshop opposite the pond in Cromford – the type of place that had vanished from most high streets, but still lurked in corners of the Peak District. Cooper could see it across the water as he entered the village. On a day like this, he’d have liked to be free to spend an hour or so browsing the shelves, making discoveries, drinking a cup of freshly ground coffee. Maybe there’d be home-made homity pie on the menu.

But he had to drive on, filtering left at the crossroads on to the A6. After the tightly clustered cottages of Cromford, Masson Mill looked enormous in its position between the road and the river. This stretch of the Derwent Valley had been classified as a world heritage site a few years ago. When the centre of the cotton industry moved to Manchester, the mills and millworkers’ villages of Derbyshire had been left almost intact in their rural backwater.

Some of the old millworkers said that the ghost of Arkwright himself still trod the creaking floorboards at Masson Mill. It was easy to believe that he wasn’t long gone when you saw the dusty boxes stacked on the shelves in the spinning room. ‘Return to Sir Richard Arkwright’. Of course, everyone knew he was buried down the road at Cromford. The mansion he’d built, but never lived in, stood directly across the river from the mill, among trees that he’d planted but never seen grown to maturity.

The back wall of the mill overlooked the river. Its five storeys were full of windows – long ranks of them separated into pairs by stone mullions. They were spaced with Victorian precision, but so small and dark that nothing was visible behind the glass. Those windows stared out across the rushing water like blank eyes. There were scores of them, a hundred pairs of eyes – a high, brick wall full of dead faces.