We set off in the direction of Park Lane. “Don’t alarm yourself,” he said, “but cocaine is a drug made from the coca tree, found in South America. It’s not used for medicinal purposes, though. People – rich people, as it’s very expensive – use it to make themselves feel good.”

I was not alarmed, but interested. “Like wine and cigarettes?”

“Exactly, except that alcoholic drink and tobacco are legal, and cocaine is illegal. You can be arrested and imprisoned for possession of it, and if you are caught importing or selling it you’re in very deep trouble indeed.”

My interest increased. “Are you telling me that David uses this thing? Cocaine?”

“Yep,” he nodded, “along with the rest of his set.”

“What do they do with it? Drink it? Smoke it?”

“No, it is taken through the nose. They sniff it.”

“And they risk getting caught just to feel good?”

“It’s fashionable.” He gave a small shrug. “It’s what the people David wishes to impress do. And like cigarettes, the more you have the more you want.” He looked at me squarely. “Believe me, Clara, I’d never touch cocaine myself. I much prefer the old cigs, and whisky, of course. But people like Marjorie Cunningham are so dependent on cocaine to keep themselves happy, they cling to David because he knows where to get the stuff.”

A memory rose up, and I gasped. “Oh!” I stopped so suddenly that a man in a top hat bumped into us. He apologized, raised his topper and went on his way while I stared at Aidan, stricken. “That’s why you thought it was funny that I should imagine Marjorie had come to David for a job. She had come to him for cocaine, hadn’t she?”

He let his expression be his answer. I went on standing there, my brain busy. “So you are hoping to photograph David sniffing this stuff?” It was like a script from a film. Trying to control my voice, trying to be as nonchalant as Aidan, I pressed on. “And … you will tell David that you’ll give the photographs to the police unless he destroys the photographs taken in the hotel?”

“Exactly.” Aidan’s eyes had begun to glow a little. He began to walk on. “We’ll catch him unawares. You’ll set up the photograph, and I’ll take it. All you need to do is what you’ve been doing for the last six months. Act.”