‘Well, damn!’ Pavarotti sounded very Welsh in his indignation. ‘The tide’s gone out.’

‘Tide be buggered!’ I snapped. ‘We’re a thousand bloody miles from the sea.’

‘Only joking. We don’t need our masks now, that’s for sure. Hardly need the suits, even. We can walk straight through with our heads above water.’

‘All the better,’ I told him. ‘But . . . hey, what’s this?’

On the right-hand wall ahead of us, just above the water line, the top of an arched recess was showing – clearly the opening to a side-tunnel. It was bricked in, but some of the cement had washed out and I could see water welling in and out through the gaps.

‘That’s where it’s gone,’ I said. ‘Or where it came in from. Part of the system.’

‘So what?’

‘So nothing. We carry on.’

And through the flood we went, moving slowly to create as little disturbance as possible. Once in the water the steel case, with air trapped round it inside the rubber bag, was almost floating, and towed along easily.

Very soon we were out of the water and at the site itself. We laid the case down a few feet short of the end of the tunnel, to make sure no debris fell on it when we started digging.

‘There you are,’ I told Toad. ‘It’s going in that recess. And there’s the shaft for the SCR. You get cracking, and we’ll be back.’

One of Toad’s unnerving features was his silence, the fact that he spoke so little. You felt that his brain was turning over smoothly like a well-oiled mechanism, but you hadn’t a clue what he was thinking. Now, as we left him, he stood there dry-washing his hands without a word.

‘I wouldn’t mind sealing the bugger down here,’ I said as we started out with our second load. ‘That’d stop him annoying me.’

By the time we returned, Toad had the lid off the case, and for the first time we got a glimpse of its contents: a terrifying maze of bright blue and white wires snaking round compartments of different shapes. He was wearing latex gloves and a pair of headphones, listening carefully as he touched a probe on one point after another. He had small socket spanners, Allen keys and battery-driven screwdrivers laid out on a mat beside him, occasionally picking one up to tighten or loosen a connection. But as soon as we delivered the SCR, he turned his attention to that, because he was anxious to have it up and working first.

Rather him than me, I thought as Pavarotti and I peeled off our dry-suits and got stuck into the digging. Secretly, though, I felt a bit like a navvy labouring in the presence of a technician who understood things that would always be beyond me.