Cooper had drawn North Parade, and had to walk back the way he’d come. There must be a dozen cafés and fish restaurants in this length of road alone: Taste of the Waves, Frankie and Joe’s, the Promenade Fish Bar . . . And then there were all the pubs and restaurants, and the big hotels on the hillside.

Between them, they accounted for the most distinctive feature of Matlock Bath, a characteristic that Cooper remembered so well from previous visits and which was still present now, undiminished by the years. It was the smell of fish and chips, hot and vinegary, hanging permanently over the promenade like a haze.

Many of the shops, amusement arcades and restaurants were closed. Of course, it was a weekday and out of season, with the children back at school for the new term. But some had opened to cater for the bikers and a few other visitors.

He passed the photograph museum, Life in a Lens, and noticed that a Victorian tea room had been opened on the ground floor. He’d have to call and ask about the webcam later.

The aquarium had a window display of a deep-sea diver and a treasure chest, but the ground floor was taken up by an amusement arcade. He asked at the pay booth for the member of staff he wanted and was directed past the slot machines and up the stairs into the aquarium.

‘He’ll be with you in a minute.’

‘Thanks.’

At the top of the stairs, he waited by the original petrifying well. This was what the first visitors to Matlock Bath had come to see – ordinary items apparently turning to stone. Thermal water still ran down a channel through the building, pumped up into sprays and gurgling ominously somewhere beneath his feet. The objects he could make out under their layer of calcium included teapots, a bird’s cage, a telephone and a deer’s skull, complete with antlers. As usual, visitors had thrown coins into the water, forever hoping for a bit of good fortune. This was one belief that seemed to survive, no matter what – an enduring faith in the magical properties of water.

Cooper wondered if Rose Shepherd had tossed a coin in here on Saturday. She sounded like a woman who’d desperately needed a bit of good luck.

Suddenly, a man was standing beside him.

‘Is it me you’ve come to see?’

‘Yes, if you called the police in response to our appeals.’

Cooper fumbled for his ID, but the man didn’t even look at it.

‘I’m a bit busy, so you’ll have to keep up with me while I’m working. I have to check all the safety precautions and signs while it’s quiet.’

‘All right.’

‘Through this way, then.’

‘Before we go any further, is this the woman you remember seeing here on Saturday?’