‘What the . . . ?’ Pav was crouching beside me on our ramp of spoil. He raised a hand. ‘Listen!’

At first we could feel it rather than hear it: a deep vibration more than a sound, a shudder so low that it seemed to come through our boots. But in seconds it built into an audible flutter, then into a rumble, then into a roar which filled the tunnel and made it shake. The water behind us had long since settled back into stillness after our passage through it. Now I saw a ripple on the black surface, and I was convinced that the roof was about to cave in.

I looked round at the concrete blocks behind us. We were trapped between the wall and the water in a section of tunnel about fifteen yards long.

The pulsating roar built up still louder until it seemed to come from right over our heads. Particles of brick dust started to fall from the roof. I looked up at the brickwork right above us, fearful that I’d see water break through the joins, expecting to be swamped any minute. I made a grab for my mask and breathing kit.

Into the din Pav yelled, ‘Fucking Metro!’

‘Bollocks!’ I shouted. ‘No Metro line anywhere near. I checked it on the street plan.’

‘Gotta be a boat, then.’

‘A boat?’

‘On the river.’

‘Some boat.’

We were bellowing at the tops of our voices. Toad stood there looking vacant, but I think he was just as scared as we were. Then I realised that the racket was diminishing, and I felt sure Pav was right: a boat had gone up or down the river, close over our heads.

After that scare, it took Toad only a few more minutes to complete his preparations.

‘OK,’ he announced, ‘we’re ready to go.’

Anywhere else, the idea of taking orders from Toad would have made me see red, but here we were entirely in his hands and it didn’t bug me at all to follow his instructions. With him directing and helping, we raised the base section of Apple – the heavier of the two – and eased it sideways on to the rough shelf we’d created. That was relatively simple. The harder part was to lift the top section, turn it over in mid-air, then manoeuvre it into position above its mate without letting the two touch or knock together until they were perfectly aligned. The second part weighed just on 1501bs, and even for two fit guys, holding that amount out at arm’s length was no picnic.