Instinctively, Cooper stepped forward. He quickly came up behind Fry and was able to grasp her arm to support her just as she began to lose her balance. She hardly seemed to notice his assistance.

‘Where do we come to if we go this way?’ she asked.

‘We’re in Staffordshire now,’ said Cooper. ‘This track leads up towards a village called Hollinsclough.’

‘Staffordshire? Really?’

‘Of course.’

Cooper kept his eye on her. Even Fry didn’t have any jurisdiction in Staffordshire. It wasn’t in the East Midlands, so it was out of the EMSOU’s patch. They had to rely on mutual cooperation with a neighbouring force.

‘It’s strange really, to think that you’ve crossed a border,’ said Cooper, turning to look back at the Corpse Bridge. ‘It’s such a narrow stretch of water and such a small bridge. Yet it’s always meant so much to people because of its position and significance.’

‘A crossing from life to death?’ suggested Fry.

Cooper swung round sharply. ‘What made you say that?’

Fry smiled again. ‘I just thought it was something you would be thinking, Ben.’

‘I see.’

‘Well, wasn’t it?’

‘Something like that.’

Fry nodded, apparently pleased with herself. Just because she’d guessed at a notion flitting through his imagination. Was that what she regarded as insight?

‘And what about this way?’ she said, pointing to the main route of the trackway where it climbed through the trees.

‘That’s the coffin road,’ said Cooper. ‘It goes to Bowden.’

‘Derbyshire?’

‘Just about.’

Cooper explained the nature of the estate village and its relationship to Knowle Abbey.

‘And that’s where the coffin way leads to?’ she asked.

‘Yes, to Bowden. It’s where they had to take their bodies, for burial in the graveyard there.’

He could see the concept was difficult for Diane Fry to understand. And why wouldn’t it be, for someone who had grown up in a city like Birmingham? To Fry, the idea of trekking across the Peak District countryside carrying a coffin probably sounded like just another inexplicable rural tradition.

Cooper showed her the route of the coffin road on his map and carefully explained why people from the hamlets to the east had been forced to carry their dead all this way. It hadn’t been their choice, or a random whim. They were completely at the mercy of those who had all the money and power.

‘But this track crosses the bridge, then recrosses the river a few hundred yards further down,’ pointed out Fry. ‘Why would they do that? It doesn’t make sense. You just end up back on the same side of the river that you started from.’

He could see Fry frowning at the map in bafflement. What she said was accurate, of course. That was exactly what the coffin road did. And she was right, too, that there seemed no sense in it. No rhyme or reason, or apparent purpose.

Or at least, no reason in the purely logical, modern world that Diane Fry lived in.

‘Spirits,’ said Cooper.

‘What?’

‘Spirits,’ he repeated, with that sinking feeling of resignation. He knew what her response would be when he tried to explain this. Derision and disbelief. But he was quite used to that now.

‘Do you really mean—’ she began.

‘Yes, spirits,’ he said. ‘Spirits can’t cross water.’

‘Oh, for heaven’s sake.’

‘Perhaps.’

Fry turned away and began to walk up the track and Cooper followed her. As they climbed away from the banks of the river, the trees soon began to close in again.

Cooper couldn’t escape the feeling that Diane Fry was observing him constantly. He supposed she was waiting for him to slip up and make a mistake. She’d be watching him for any sign of weakness or hesitation, an indication that his mind wasn’t fully focused on his work, that his powers of concentration still hadn’t fully returned. She’d be hoping that he wasn’t up to the job.

‘Shrieking in the woods, white figures moving through the trees,’ she said. ‘What would the folklore say, Ben?’

If he’d been talking to anyone else, Cooper might have mentioned corpse candles. It was the name given to a flame or ball of light seen travelling above the ground on the route from a cemetery to a dying person’s house and back again. For some reason the light was usually blue. A similar light appearing in a graveyard was believed to be an omen of approaching tragedy. Cooper seemed to recall that they appeared on the night before a death. The stories told about corpse lights were like those of the will-o’-the-wisps, mischievous spirits who attempted to lead travellers astray.

As Diane Fry would certainly have pointed out, there were always logical explanations for these things. Anyone observing a will-o’-the-wisp might be seeing nothing more than a luminescent barn owl. A wildlife officer had once told him that some barn owls possessed a form of bioluminescence caused by honey fungus. The white plumage of the birds could look eerie enough at night, if you glimpsed one in your torchlight. A luminescent barn owl flitting through the darkness would be enough to spook anyone. And corpse candles? Witnesses might just have been noticing the effect of methane gas, the product of decomposing organic material in marshes and peat bogs.

But even in the twenty-first century, the prosaic scientific explanations weren’t always what people wanted. Everyone liked a bit of mystery. Generation after generation, the more superstitious inhabitants of Derbyshire had preferred to believe in spirits.

‘Who told you that anyway?’ asked Cooper.

‘DC Villiers. I heard about the statements from your members of the public this morning.’

‘Oh.’

‘Have you got a problem with that? We’re colleagues, aren’t we? We should be working together.’

‘If you say so.’

Cooper stopped. He’d caught a glimpse of something blue glittering among the trees, a flash of light, as if from a piece of glass reflecting the sun. The sight was irresistible, a signal tempting him from the path. He had no option but to turn aside and investigate.

When he got closer he could see that what he’d seen was a ball of smoky blue glass, the kind of thing sold in craft centres and gift shops for use as a table ornament or a flower vase. His sister would have taken it home and placed a scented candle inside it.

But inside this one was a tangle of threads. There were lengths of cotton of every colour – not only white and black threads, but bright strands twisted among them in no discernible pattern. It was just a random hotch-potch of colour, all given an eerie glow by the blue of the glass. The neck of the ball was attached to a branch of a rowan tree by a pair of ribbons and it moved slightly in the breeze, spinning one way and then the other. It had been placed at a height just above Cooper’s head, but he could reach up to stop its movement. Then he saw the scraps of paper entangled among the threads, rolled into little tubes and thrust into the multicoloured mass.

‘What is it?’

Cooper turned at the sound of Fry’s voice. As happened so often, her words intruded like a cold dose of reality from the outside world at a moment when he was contemplating the mysteries of the rural imagination, feeling the centuries of belief in magic running disturbingly through his veins. There was something about these old superstitions that made him shiver, not only with apprehension, but with understanding too.

‘You don’t want to know, Diane,’ he said.

‘I suppose that means it’s something absurd and rustic.’

‘Well, it’s a witch bottle,’ said Cooper.

Fry snorted. ‘Exactly.’

Cooper looked at her, not at all surprised this time that she’d noticed him leaving the path and decided to follow him. It was like being under twenty-four-hour surveillance. He wondered what she would have done if he’d simply sneaked off to relieve himself behind a tree. Would she have stood there making notes?