I heard him scraping on the brickwork as he descended, then felt him touch down beside me. The moment he let go of the ladder, the end went snaking up as Toad reeled it in. His brief was to seal us down with two spare locks he’d been carrying, then to hide up somewhere close by until the time came to release us. That way, if by any thousand-to-one chance somebody did come along to check the padlocks, he’d see nothing amiss. Toad would be in radio contact with Whinger throughout, and could call him in to lay on a diversion if anything started to go wrong.

When I heard the cover come down with a faint thud, I felt a shudder of claustrophobia run through me. If anything serious befell Toad and Whinger, we’d be sealed down here for the duration. Pav was obviously having the same panic, or worse: I could hear him breathing deeply and effing and blinding under his breath.

The air was rowsty and moist, full of a smell of damp decay. Our head-torches revealed a tunnel with a horse-shoe section, lined with bricks. The roof was just high enough for me to stand upright, but Pav, who was a couple of inches taller, had to crouch slightly to keep his head clear.

Somehow – perhaps because of the colour of the Kremlin walls – I’d expected the bricks to be red. In fact they were dirty cream, or had been: much of the surface was black with fungus or slime, and when I touched the wall beside my shoulder my fingertips slid along the wet surface leaving pale streaks. In many places individual bricks had crumbled or fallen out, so that there were frequent piles of rubble on the floor. That gave me encouragement; if the tunnel had been in immaculate condition, any tampering we did would have been that much more obvious.

I bent down and examined the floor. It was evenly covered with damp dust – paste, almost – the same dull colour as the walls. There was no sign of any disturbance – not even any traces of rats, which I’d expected to find. I saw that we wouldn’t be able to help leaving footprints.

We’d measured the distances, and I had them in my head: 160 metres to the river bank, 110 metres across the river, seventy-five to the Kremlin wall: 345 metres in all to our preferred site. When I went forward I was going to count.

‘Ready?’ I whispered.

Pav didn’t answer.

‘Eh!’ I went. ‘Let’s go.’

‘You go!’ he gasped in a peculiar voice. ‘I’m staying here.’

I could tell he was having problems just from the way he sounded. When I put out a hand and touched his arm, I felt him shaking violently. I turned the beam of my head-torch on his face and saw beads of sweat trickling down his cheeks.