Turning her attention to the lower hinge, she repeated the process, ignoring Derek Knight’s frantic yells. This time, she pulled the pin completely clear, then worked the top pin free. Now, only the lock held the door in place. Gripping the edge of the door with her fingertips, Lindsay inched the hinge side of the door back from the frame. The tongue of the lock creaked against its socket, but she managed to pull the door back sufficiently far to clear the jamb. Then, one hand on the handle, the other on the hinge side of the door, she slid the whole thing sideways, pulling it neatly free of the lock.

Derek Knight was standing in the hall facing her, his mouth open and his eyelids as wide as they could go without surgery. Lindsay stepped through the doorway, leaning down to pick up her backpack. “Sorry, got to go,” she said.

He lunged at her, mouthing something incomprehensible, but Lindsay sidestepped neatly and rushed for the door of the flat. She took the stairs two at a time, the blood pounding in her ears, obscuring any sounds of pursuit. She didn’t even bother closing the street door behind her, sprinting down the street in the opposite direction to the tube station. At the corner, she turned left at random, cutting diagonally across the road and jinking into a mews court that ran between two parallel streets.

At the end of the mews, she stopped running. She wasn’t dressed for jogging and no one on the Islington/Canonbury border ran except joggers and muggers. As she turned left into the next street, she heard the whooping sirens of police cars nearby. On the corner was a pub. Lindsay breathed deeply to calm her thudding heart, walked straight through the doors and ordered a pint of bitter.

The first drink had gone down so well, Lindsay hadn’t had to work hard to persuade herself that she deserved a second. She’d found an unobtrusive corner, hidden by a raucous group of youths wearing sweat pants, sports shirts and training shoes that had never seen activity more strenuous than the game of darts their owners were throwing. Lindsay kept her head down and thought about the little she’d learned from Derek Knight before she’d given herself away in so embarrassingly inappropriate a fashion.

What stuck in her mind were his comments about the doors. From the moment Sandra Bloom had revealed that Penny’s death was murder, Lindsay had recognized it as a carefully planned, premeditated crime, based as it was on the plot in Penny’s own book. According to Derek Knight, the flat door was ajar, but not flung wide, which tied in with that supposition. It wasn’t left that way in a panic, but deliberately. It also indicated that the killer wanted the body to be found fairly quickly.

However, the mortise lock on the street door had been left undone. That suggested either that the killer didn’t know the residents routinely kept it locked or that in his or her haste to get away from the scene of the crime they hadn’t been able to find Penny’s keys. It was confusing. On the one hand, it had been made to look like an accident; on the other hand, like murder.