‘Ah, there she was struggling a bit, poor lass. She says they’d come in earlier, a man and a woman. But she had no reason to take particular notice of them. The Shepherd woman came in about a quarter of an hour later, and she was on her own, which is more unusual. She ordered a coffee, paid for it, then took her time looking round, and went and sat at the couple’s table.’

‘Did she seem to know them?’

‘That’s what Tina’s not really sure about. There were no empty tables, so Miss Shepherd would have had to sit with someone, and she chose those two.’

‘Right. We don’t know why, though?’

‘Maybe because they looked the most harmless. All Tina can say is that when she took the coffee to the table, the three of them weren’t talking and the atmosphere seemed cool. But they did chat a bit later on. The couple left the café first, and Miss Shepherd went out right after them. The money for the couple’s bill was left on the table.’

Cooper unlocked the car. Standing at the kerb behind it was an entire family of bikers – mum, dad and two small children, all dressed in matching leathers and gathered round a pair of purple Suzukis.

‘Well, it’s something at least, Gavin,’ he said. ‘She must have come down into Matlock Bath for a reason.’

‘Oh, and I did a couple of shops,’ said Murfin.

‘Yes, the ice-cream parlour. I saw that.’

Murfin groaned theatrically. ‘You know, Ben, you’re getting as bad as Miss.’

‘Get in the car, Gavin. We’ve got to call at Masson Mill.’

Masson had been the world’s oldest working textile mill until production stopped fifteen years ago. Here, in the middle part of the Derwent Valley, was where industrial history had changed. It had all started for Sir Richard Arkwright at Cromford Mill, just downstream. But Masson was his great flagship.

Cooper couldn’t remember details of the innovations that led to Arkwright’s success, the industrial secrets German manufacturers had gone to great lengths to get hold of. But he could see how Arkwright’s status had risen purely by looking at the building. This mill hadn’t been built, but designed. Instead of a dark, cavernous shed, it was an edifice intended to impress. The three central bays were built out towards the road and decorated with half-moons of glass between Venetian-style windows. Above the windows stood a shuttered cupola, and Sir Richard’s name spelled out on the brickwork in proud capital letters.