‘Somewhere there ought to be an “Aus”,’ said Fry. ‘Let go.’

‘Yes,’ said Cooper, searching for clues to whether he should be laughing or not. He marvelled at the change that had taken place in her. For a short while this afternoon, he thought he’d got close to the real Diane Fry. But she’d slipped away from him and he’d lost the scent. And that was wrong, because this was his territory, not hers.

Cooper gazed around. You could see Longstone Moor quite clearly from here. On these moors there were still traces of the old packhorse ways that had once been used by travellers such as pedlars, tinkers and badgers. The inhabitants of one valley often knew nothing of the neighbouring dale, because they were separated by inhospitable moorland. For directions, they had only rock formations with descriptive names, or ancient man-made features. Head for the Eagle Stone, turn left at Hob Hurst’s House.

And the biggest problem was the weather, which changed so suddenly on the moors. Low cloud, heavy rain, fog, snow – they could all reduce visibility so much that travellers often lost their way during the winter, and that meant losing their lives. Just as Philip Worsley had on Wednesday.

The landscape was scattered with the small white dots of sheep, a typical White Peak sight. But here and there among them, like alien giants, were much larger objects, bright yellow, their long necks swinging as they hunted backwards and forwards, the scream of their alarms reaching him on the wind. Quarrying machines, hacking out more limestone to fill the dumper trucks. Soon, this landscape would be gone for ever.

‘And what about Michael Clay?’ asked Cooper. ‘Is there a woman involved there, too?’

Fry frowned. ‘Well, not his niece, Pauline Outram. And not his daughter, either.’

‘Could there be some other local connection then, apart from Pauline? What if there’s a real significance in the Royal Observer Corps stuff?’

‘But Michael Clay didn’t serve in the Observer Corps,’ said Fry.

‘No.’

Cooper recalled the photo of the ROC members. Their blue battledress tunics, their berets at rakish and unflattering angles, their cap badges glinting. It had been a colour photo, so he’d guessed the date to have been the sixties. In the photo, the ROC crew were standing in front of a small brick tower with no roof and a set of steps up the outside wall. An aircraft-monitoring post. According to David Headon, the last overground structures had been abandoned when the RAF no longer required the ROC for that purpose.

Then he realized what he’d been missing. When he’d first spoken to Headon about the ROC, he’d mentioned cutbacks in the Corps. But he hadn’t been talking about the final stand down in 1991. That was Cooper’s own mistake, a wrong assumption.

So what was it that David Headon had said, exactly? ‘There were thousands of us, until the cutbacks started in the sixties.’