A railway line – used only for tourist rides in the summer – ran along the edge of Mannez Quarry, about twenty minutes’ walk from The Lookout. The line split into three and continued down to a large shed which housed the diesel engine and carriages that made the journey to Braye Road Station, twice a week on Saturdays (I had seen flyers advertising the trip back at the hotel). My first thought, as Hawthorne and I drew up, was that despite the blue water and an abundance of bright yellow gorse sprouting all around, it was a remarkably desolate, even a grim place. Perhaps it was The Odeon, the naval tower built by the Nazis, that inspired my sense of unease. It stood high above, as sinister a building as I had ever encountered, with its three dark slits dissecting the grey concrete. From the day of its construction it had been a witness to murder and barbarity. It was somehow fitting that this should have continued into the twenty-first century.

Underneath it, there was a cave, presumably man-made and used to transport food supplies or munitions. It would be easy to miss. There was no sign, no indication that it existed apart from a flattening of the grass towards the entrance. I had been unfair to the sniffer dogs. Apparently, one of them had barked to be let free and had then run inside.

Torode had rung us with the news and he was waiting for us when we arrived. ‘In there!’ He looked tired and ill. He didn’t say anything else.

We made our way towards it, passing the rusting skeleton of an old carriage that had been abandoned in the long grass. Old, abandoned machinery can have a certain beauty, but this was different. Maybe it was just the mood I was in, but somehow the broken, twisted metal unnerved me: a ghost train in every sense of those words. I was filled with dread as we reached the mouth of the cave and went in, leaving the light behind.

The roof was high enough for us not to have to bend down. The ground near the entrance was covered with rubbish: old tyres, crates, coils of wire. From there, the cave continued a surprising distance into the cliff and we would have needed torches, except that the police had already rigged up a looping series of electric bulbs, dangling from the walls.

Helen le Mesurier was about halfway to the end. Just as we reached her, there was the flash of a camera and I saw her as if in a photograph, an image frozen in the brilliant light.