The shelter proved to be not much more than a tunnel driven horizontally into a piece of sloping ground – a primitive structure with an arched roof of corrugated iron which was about ten feet high in the middle and dropped down to ground level on either side. From the front we could see that the tin was only the inner lining: on top of it was a layer of concrete maybe a foot thick, and then earth. In the front wall, made of concrete blocks, was a small opening at shoulder height, designed to let in light and air, and the entrance was to one side. Since the only illumination came through those two apertures, the inside was dark as a cave and we had to feel our way past the edge of the heap of old planks dumped in there.

‘Should have brought a torch,’ Whinger muttered.

‘Yeah – but we can hack it enough for now.’

The shelter ran about thirty feet into the side of the hill and was all one space – no divisions. When our eyes had adjusted to the gloom we could see that the wall at the back was like the one at the front – concrete blocks – but in less good nick: damp had worked its way through, cracking the mortar and producing dark stains. Pressure from the earth behind had pushed two or three of the blocks forward so that they stood proud of their neighbours. When I ran my fingers down the wall they came away wet and smelling faintly of iron.

‘What we should do is get behind the blocks and dig out a hollow,’ said Whinger. ‘Then put the wall back up. With the blocks loose like that, it’s a piece of cake.’

‘It would be if we didn’t have to dispose of the spoil.’ I bent down and scuffed my hand over the floor. ‘Feels like bare earth. But the stuff coming out of the bank’s bound to be a different colour.’

‘Yeah – but who’s coming in here to see it?’

‘A hundred to one, nobody. But if somebody did we’d be buggered. Better to get rid of it. Let’s recce a dump site outside.’

Back in the open, we found an ideal place within thirty metres of the entrance: pushing through scrub, well away from the track, we nearly fell into a deep pit with gorse bushes growing over it. Sand or earth dumped through the branches would vanish into the hole, which would have taken tons and was far bigger than we needed.

‘That’s it, then,’ I said. ‘When we come, it’s going to be all hands to the pumps. We’ve got to do the whole job in one night: drop the wall, dig the recess, place the device, rebuild the wall, skim it with mud to mask the new joints, and away.’