I closed the valve and allowed time for Whalley to inhale the sleeping gas into his lungs. I waited some moments, then opened the valve again for a five-second burst. A sudden blast of cold air, accompanied by the noise of rushing wind, told me Whalley must have wound down his window. No one does that on a cold night unless they are getting tired.

I was a little concerned about the gas. It was the first time I had used this particular type, fresh off the production line of the Playroom – the dirty tricks department of British Intelligence. Messrs Trout and Trumbull, its inventors, claimed it was the first non-inflammable sleeping gas in the world; not having used it before, I had no real idea of its strength.

I waited the minute and a half it took him to decide he had had enough fresh air and wind up the window, which he did, then opened the valve again, this time for eight seconds. Again he opened the window. I hoped to hell he would do the sensible thing and pull over and stop, and not fall asleep at the wheel with an articulated lorry coming the other way.

The window went up again, and I released another five-second burst. This time, the car slowed noticeably. Suddenly he braked very hard, the car swerved, and I felt a bumpy surface under the wheels. We carried on for some yards, then stopped, and Whalley switched off the engine. He yawned loudly, and I released another burst as he yawned. There was the click of a lever in front of me, and the driver’s seat tilted back a few degrees. Whalley was sound asleep before he had even leaned back into it.

Everything was quiet for a few moments. There were some clicking sounds from the engine as the coolant dribbled through the pipes, the roar of a lorry in the distance, then the heavy rhythmic breathing of Whalley. It was pitch dark, we were either in a lay-by or on the grass verge. I put the valve right under Whalley’s nose and opened it for four seconds. According to my instructions, he would wake up in a couple of hours, feeling very thirsty and slightly queasy – neither sensation being particularly out of the ordinary for someone who is attempting to shake off fatigue by sleeping in his motor car.

A lorry thundered past and the Cavalier shook. I switched on the car’s interior light, and right away saw a small Jiffybag on the front passenger seat. Fortunately, it had not been sealed, and I removed its only content: a thick plastic container. I pulled out my torch and shone it on the container. Nothing was written on the outside, but I knew what it would contain. I had frequently seen such containers before. I opened it, and I was right: it was a video-cassette.