We’d been through all the measurements again, and I’d calculated that the fully flooded section of the tunnel couldn’t be more than fifty or sixty metres long. So I wasn’t surprised when, after two minutes half-walking, half-swimming, my head broke the surface again. As we continued to advance, an upward slope lifted us steadily clear of the water. Soon we were back on dry land.

The orginal distances given us by the Firm turned out to be spot-on. A total of 340 metres from the old stable, we came to a circular hole in the roof – the ventilation shaft. When I stood upright with my head in the bottom of it, my helmet lamp revealed that it did not rise vertically, but turned at an angle to my right. I could feel cool air flowing down, so I knew it was open at the top.

‘Shit hot,’ I told Pav. I brought out my tape and held it across. ‘Twenty-eight inches. That’s easily big enough to accept the SCR – and anyone making visual checks down the manhole won’t be able to see round this corner. Made to measure.’

Five metres beyond it, the tunnel had been sealed with a wall of concrete blocks. Yet providentially, just on our side of the barrier was another big fall.

‘Look at that,’ I said to Pav. ‘Made to measure again.’

‘Yeah – and we won’t even need to move any spoil. We can just add whatever we bring out to the heap that’s here already.’

We’d prearranged with Toad that we would stay down for ninety minutes. That gave us an hour of work-time, so we stripped off our dry-suits and took turns to put in concentrated attacks on the clay subsoil. Soon we were both in a muck sweat and having problems with our breathing, perhaps because the air was so damp. None the less, before our hour ran out we had enlarged the cavity to about half the size we needed. We kept the overhanging roof and edges rough, and left a pile of rubble on the base of the hole so that, when we returned to install Apple, all we’d have to do would be to enlarge the hole, clear the bed and lift the components about two feet from the floor before pushing them sideways into their final resting place.

Our return to the surface posed no problems, and once again the pick-up went without a hitch.

‘So it’s a foot on the brake, is it?’ Whinger asked as we drew away.

‘What’s that?’

‘Piece of cake.’

‘I wouldn’t call it that. But it’s possible – wouldn’t you say, Pav?’

‘Oh, yeah,’ he agreed. ‘It’s definitely on.’

So we drove back, feeling quite chuffed.