‘Oh, sh—!’

‘Sorry!’ called Headon. ‘We should have warned you to watch your head on the counterweight. If you go down in a crouch, it catches your back, and if you straighten up it gives you a crack on the skull. Are you OK?’

‘Yes, I’m fine,’ said Cooper, though he felt anything but. There’d be a lump on the back of his head tomorrow morning.

He landed at the bottom of the shaft with a small splash. Falconer was right, there was only about an inch of water, not enough even to wet more than the soles of his boots.

‘Are you coming down?’

‘One of us should stay up on top, for safety,’ said Falconer. ‘We don’t want the hatch blowing shut, do we?’

There was just a moment then, as Cooper looked up at the two faces silhouetted against the sky twenty feet above him, when a small spurt of panic ran through his chest. He couldn’t make out the faces of the two men well enough to see what their expressions were, or whether they were exchanging that secretive little glance.

‘I’ll come down with you,’ said Headon. ‘I’m more appropriately dressed.’

Cooper waited at the bottom of the shaft while Headon joined him. There wasn’t much room for two people standing on the grille of the sump. The handle of a pump protruded from the wall, and Cooper shone his torch on it.

‘That hasn’t worked for years,’ said Headon. ‘None of them do, now. They always seemed to be the first thing to seize up.’

‘It worked the sump?’

‘That’s right. You filled the priming point there and pumped the handle. My God, you had to pump hard, though, if you had a wet post. There might have been three or four hours of pumping to be done on a full exercise. One of the lads reckoned once that each stroke of the pump shifted about half an egg cup of water.’

‘Well, the exercise kept you warm on a cold night,’ said Headon with a laugh over their heads.

‘Some posts suffered so much from water that their crews had to bail them out with a bucket lowered down the shaft on a rope. You can imagine how comfortable those places were.’

In a cupboard was an Elsan toilet, like a big green metal can with a plastic seat, still permeated with that distinctive odour of the thick, blue chemical. On a shelf stood a brush and a tub of Glitto – whatever that was. A ventilation louvre stood partly open over the toilet.

Headon opened a door still labelled with a ‘no smoking’ sign.

‘This is the monitoring room. And that’s it, really. A full tour of the facilities.’

The room was so low that Cooper felt he ought to duck to avoid banging his head on the ceiling. A table and some drawers stood against the side wall, with a couple of folding chairs. Cabling ran along the wall and vanished upwards, through crumbling polystyrene tiles.