I screwed the cover back onto the light and slipped into the first lavatory on the left. I shut the door behind me, but didn’t lock it, and put my tool box on the floor. I took out a large adjustable spanner, wound it open until it fitted over one of the four bolts that secured the steel lavatory to the floor, and wound it tight onto the bolt.

The bolt wouldn’t budge at first, and a small amount of thread came away, but then it slackened and began to turn easily. I worked through the other three, lifting them out in turn and putting them carefully to one side; then I lifted up the entire lavatory, to reveal a black hole, about eighteen inches in diameter, which stank worse than the breath of a London Underground commuter. I put the seat down in the tiny space beside me. There was a sucking and gurgling, which told me the sewage tender had begun pumping the tank out.

At least the aircraft boffin at Combined Central Information knew his stuff. His tiny office was crammed full of model aeroplanes, and he looked more like an errant schoolboy than the three-times decorated Squadron Leader and fighter-pilot ace he had been in the Second World War. Reginald Braithwaite was known to the whole of British Intelligence as Biggles, and those that ever had dealings with him discovered one thing: there was nothing he didn’t know about aircraft. He had assured me that the Illushyn had a manhole directly underneath this lavatory seat, for inspection purposes, and he had been right; he’d also told me I would need a gas mask, an aqualung, and a wet-suit, and on that he was also right.

I already had the wet-suit on underneath the boiler suit, which I peeled off, and I removed the rest of the kit from the bottom two layers of the tool box; the kit included a stubby weapon with a bulbous magazine that looked not unlike an early Sten gun, and another weapon that resembled a Very pistol, and clipped them both onto the belt of my wet-suit. Then I took a bolt cutter, and snapped off the ends of the four bolts, so that they would lie in their holes without protruding, and look, to anyone that did not try to turn them, as if they were holding the bowl to the floor.

I waited until the tender had finished pumping, and then I put on the gas mask, pulled up the hood of my wetsuit, pulled on a pair of rubber gloves, packed up the overalls and the bolt-cutter into the tool box, and squeezed down into the hole.