‘Seven feet wide and sixteen feet long. Like a giant coffin, we used to say.’

When Cooper pointed his torch at the far wall, he saw two bunk beds, their mattresses still wrapped in damp plastic, with a second sliding ventilation panel over them. And Headon was right. That was pretty much it. Except for a smell of abandonment and neglect.

‘All the operational equipment was taken out, of course,’ said Headon. ‘On the wall there, you can see the fitting for the bomb-power indicator. The blast pipe was attached to a baffle assembly upstairs. And that hole in the table is where we had the fixed survey meter. That was the fallout radiation sensor, which measured the level of gamma radiation outside. The only other measuring equipment we had was the ground-zero indicator, and that was up top, too.’

‘What was that supposed to do?’

‘The GZI? It recorded the height and direction of a nuclear detonation, so we could report exactly where a bomb had gone off, and whether it was airburst or groundburst, which made a difference to the fallout.’

‘And that’s all you could do?’

‘It was a simple idea. The only trouble was, someone had to go outside to get the readings off it.’

Cooper realized there was some rubber sheeting on the floor, squelching as his weight squeezed out the water.

‘That’s conveyor-belt rubber. It was donated by the National Coal Board some time in the eighties. That’s all the insulation we had, apart from the polystyrene ceiling tiles.’

After only a few minutes, Cooper was glad to get back up into the daylight. He couldn’t imagine staying down there all night, with the hatch closed and nothing more than a dim six-watt bulb to see by. Let alone being trapped down there for the duration. Trapped inside for – what was it? – fourteen days, until it was considered safe to come out? You could go mad down there in fourteen days.

‘Seen enough?’ asked Falconer.

‘Yes, thanks.’

He got clear and watched Falconer re-fix the padlocks and turn the Allen key in its slot.

But even when the hatch was shut and locked again, Cooper still had the feeling that there was something he was missing.

‘WELL, YOU FINALLY meet a decent bloke, and he turns out to be a murderer,’ said Naomi Widdowson.

Fry looked at her. ‘There’s something wrong with the logic of that sentence.’

‘Well, what I mean is . . . he seemed all right, anyway.’

Naomi was being transferred from the custody suite at West Street to a cell on remand. Magistrates’ court would decide whether to bail her on Monday. She had been issued with her personal belongings at the desk and was waiting for the van to pull up in the yard.

‘He hasn’t been convicted yet,’ pointed out Fry. ‘In fact, he hasn’t even been charged.’

‘Yes, but you must be sure that he did it, right?’

‘We can’t comment on that,’ said Fry.