Choosing the stairs over the Gothic death trap, he started to climb. The stairs were worn and creaky with age, spiralling up around the central lift shaft. The first-floor landing had apartments numbers 1 to 3, the second floor numbers 4 to 6. By his reckoning that made number 11 the middle door on the fourth floor, right at the top of the building.

As he headed towards the third floor, Ben heard the rattle and judder of the lift descending, sounding like it was going to shake itself apart and bring the whole building down, and he was glad he’d taken the stairs. Through the wrought-iron bars he saw the lift’s passenger, a lone man making his way down from an upper floor. Ben gave him only the briefest of glances, but his eye was trained to notice details. The guy was standing with his back to Ben and his face turned away. He was broad-shouldered and well built, about Ben’s height at a shade under six feet. He wore black leather gloves and a long dark coat, quality wool, expensive, with the collar turned up. His hair was short and black, silvering in streaks. Ben caught a whiff of aftershave. The man didn’t turn around as the death trap rattled on its way downwards.

Ben watched the lift disappear below him between floors, then kept on climbing the stairs. A strange, vague feeling had suddenly come over him, as though something at the back of his mind was needling him. He had no idea what it was, and quickly forgot about it.

Moments later he reached the top floor. As he’d guessed, apartment 11 was the middle door of the uppermost three apartments. He paused on the landing for a moment, thinking of the most innocuous way to introduce himself. Honesty and openness were the best policy. She would soon realise he was the friendliest and least menacing guy on the planet. At any rate, he could be that guy when he wanted.

He removed the handwritten note from her phone case, since he’d no longer need it. Then raised his hand to ring the doorbell with a knuckle. Force of habit. In his past line of work, leaving fingerprints often wasn’t a good idea.

Then he stopped. Because he’d suddenly noticed that her door wasn’t locked. Not just unlocked, but hanging open an inch. He used his fist to nudge it gently open a few inches more, and peeked through the gap. The apartment had a narrow entrance passage papered in tasteful pastel blue, with glossily varnished floorboards. There were four interior doors leading off the hallway, one at the far end and one to the left, both closed, and two more to the right, both of which were open though Ben couldn’t see into the rooms from where he stood.