‘Sometimes. There are a few enthusiasts, or old observers. You have to be careful, though. You should always go with someone.’

Headon pointed to a fenced section of ground with a small gate. There certainly wasn’t much to see.

‘On the surface, there’s the shaft, of course,’ he said. ‘That’s where the ground-zero indicator was mounted. Over there is the ventilator turret, and the mounting point for the fixed survey meter, with the top of the blast pipe near it.’

‘What’s left inside the bunker?’

‘This one still contains the bunk beds, mattresses, chairs, kettles, a few other odds and ends. We had to be self-sufficient, you know. If fallout did occur near a post, the observers could hardly pack up and go home. It could have taken a couple of weeks for the air outside to clear enough for the crew to be relieved. These posts were designed to close hatches to the outside world until the danger passed.’

Headon stood by the hatch at the entrance to the shaft and patted the concrete with a gesture of fondness.

‘I was number three observer,’ he said. ‘It was one of my jobs to climb up through the hatch and sound the siren when Attack Warning Red was received.’

‘Attack Warning Red?’

‘The warning of an imminent nuclear attack. Attack Warning Black was the fallout alert. Strike command would pass a warning to the carrier control points in police stations, then it was transmitted across the network to activate the sirens.’

‘If the receiver hadn’t been accidentally left on and flattened the battery,’ said Falconer.

‘Yes, provided the circuits hadn’t been knocked out by a thunderstorm. There was absolutely no EMP protection.’

They both laughed, sharing their hilarious memories of British incompetence in the face of a nuclear holocaust.

‘As number three, I also had to look after the ground-zero indicator,’ said Headon, ‘which was a sort of bread bin with four pinhole cameras. That meant coming upstairs. It would have been the most dangerous job of all, if the balloon had ever gone up.’

‘Upstairs?’

‘On to the surface.’

‘We were part of 8 Group, based in Coventry,’ added Falconer. ‘But posts were organized in clusters. Edendale was post 8/A5. We had Buxton to the west, but the other two posts in ‘A’ cluster were out at Beauchief and Wickersley.’

‘Those are in South Yorkshire,’ said Cooper. ‘So each cluster covered a pretty big area, then.’

‘Yes.’

Cooper thought he detected a momentary exchange of glances between the two men. But it was very brief, and he didn’t have a clue what it could mean. He’d probably imagined it, anyway.

‘Were there other local clusters?’

‘Of course. B-Cluster covered the Derbyshire and Nottinghamshire border – Ashover, Whitwell, Farnsfield. There were two more clusters in the south of the county.’