Cooper located the places mentioned on his map. Each of those other clusters had a post not too far away, but their coverage spread outwards to the east, west and south. A-Cluster looked odd, though – the three posts were pretty much in a straight line from Buxton across the southern outskirts of Sheffield and Rotherham. The line ran right through Eyam.

‘Just four posts in A-Cluster?’ he asked.

‘Oh, yes.’

‘Was there anyone called Clay in your post crew?’

They both shook their heads. ‘No, not here,’ said Headon.

‘Or Outram?’

‘No, sorry.’

Finally, Cooper showed them a copy of the photograph taken from Michael Clay’s laptop case.

‘Not our crew,’ said Headon.

‘No,’ agreed Falconer. ‘The big chap looks familiar, though.’ But then he shook his head. ‘I probably saw him at an annual camp somewhere.’

‘You don’t recognize the building?’ asked Cooper.

‘Those Orlit posts are long gone. They were for aircraft recognition.’

Disappointed, Cooper looked down at the green-painted hatch cover on top of the shaft, and the ventilator turret close beside it.

‘Is it possible to take a look inside the bunker?’

There was that glance again. He was sure of it this time. Something they didn’t want him to see down there? A locked underground bunker in the middle of nowhere presented all kinds of opportunities. He was just starting to run through them in his head when Falconer nodded.

‘Yes, OK.’

‘I can?’

‘There’s no light down there, though. We only ever had a battery for power.’

‘That’s all right. I’ve got a good torch in my car.’

Falconer produced a set of keys and opened the padlocks on the hatch. Then he inserted a narrow rod like an Allen key into a slot on the cover and twisted it. The iron cover lifted on its levered hinge much more easily than Cooper had expected.

‘There’s a counterweight,’ said Headon.

‘So I see.’

The mechanism looked old, and rust was showing through the green paint in patches. But it worked easily enough, so someone must have done a bit of maintenance on the post in the last eighteen years.

Cooper mounted the step and looked into the shaft. A metal ladder ran vertically down for about twenty feet, and in the light from the open hatch he could see oily water glimmering at the bottom through the mesh of an iron grille set into the floor.

‘There isn’t too much water,’ said Falconer, peering over his shoulder. ‘This was always a dry post – not like some of the others. They could flood right up to the shaft if you didn’t pump them out regularly. That’s the sump you can see there. You won’t get your feet too wet.’

‘I’m not bothered.’

He clambered gingerly over the edge and found a rung of the ladder with his foot. There wasn’t much room in the shaft, and anyone overweight might have had a bit of trouble. When he’d climbed down a few feet, he looked up again at the sky, only for something heavy to hit him hard on the back of the head, making him see stars for a few seconds.