The sixties, of course. But if there had been a big reduction in the number of observers four decades ago, then there must also have been a cut in the number of posts. Cooper pictured the map again, with that imaginary line running across Derbyshire and South Yorkshire. It was a big area for a cluster of observation posts to cover. And it wouldn’t be the way you planned it, if you were creating the network from scratch. It was more the sort of arrangement you’d end up with if some of your posts had been closed. The result of cutbacks.

The other clusters in the county all formed triangles. That made sense. So why wasn’t it the case in the north of the county? Well, he suspected it had been, once. A trio of posts – one at Buxton, and a couple of others around, say, Hope and Eyam.

So, if he was right, where was that closed post?

Cooper found himself standing above Birchlow, looking down on some of the land belonging to Rough Side Farm. Near the top of the slope was that strange hump that he’d taken for the remains of an Iron Age hill fort, or the capped shaft of an ancient lead mine.

Then he saw the line of truncated telegraph poles next to the site, and realized he’d been wrong. History was all around him in this area, yes. Thousands of years of it, dating all the way from the Romans and the Neolithic stone circle builders. But not all of its history went back quite so far.

He looked round for Fry. ‘Diane, will you come with me to see an old friend?’ he asked.

She automatically looked at her watch. ‘Well . . .’

‘Unless you’ve got something better to do, of course.’

DAVID HEADON NODDED over his glass of beer in the pub. ‘Yes, a lot of ROC posts closed in 1968. Late in the year, it was. October. The cutbacks, like I said. It was 1968, after all. A Labour government, Harold Wilson – you remember?’

‘Well, no, I don’t.’

Headon squinted at him. ‘You can’t remember what the sixties were like. You’re not old enough. I forget that you young people weren’t even around then. You missed something, you know.’

‘Music, drugs and sex, right? The Beatles, LSD and miniskirts.’

‘Bollocks. That’s just the PR. The sixties always had a lot of good public relations men, I’ll say that for them. They have a much better image than the fifties or seventies. Well, it wasn’t like that.’

‘That’s a bit disappointing.’

‘Look at this place,’ said Headon. ‘Do you think it was all sex and drugs and rock’n’roll here in the sixties?’

‘Well –’

‘Was it buggery. If you wanted all that stuff, you had to go to London. The Clappergate bus shelter wasn’t exactly Carnaby Street.’

‘The Clappergate –?’