The main entrance and stairway had once been accessible from the street, but an armed raid in the late eighties had led to this being boxed in with reinforced glass, and the large plate-glass door could only be accessed via a secure video entry system.

Observing from another Internet cafe on the opposite side of the road, Simone had tried to work out how to get inside. The obvious way would be to wait for someone who lived there to go in or out. This rarely worked for other people: on two separate occasions, she’d seen deliverymen being blocked from entering by elderly residents on their way inside. The residents had used a plastic key fob to open the door, pressing it onto a square contact pad under the grid of doorbells, releasing the lock.

This had worried Simone. She was good with locks, but it would be hard to get one of the plastic key fobs without questions being asked. Or without a mess to clear up.

Then, at 2 p.m., she’d observed a gaggle of old ladies emerging through the large glass doors, each holding a rolled-up bath towel. They’d waddled off across the grass courtyard and through a door at the back of the U-shaped building. An hour later, they’d returned with wet hair, chatting and ambling across the sunlit grass, pressing their key fobs to open the main door.

Simone had Googled ‘Bowery Lane Estate’ and had seen that there was a small council-run swimming pool on the ground floor. Four days a week it held an ‘over-sixties’ swim.

With this in mind, Simone waited until the timings worked. She followed the man to one of his regular sauna sessions in Waterloo and then doubled back to the Bowery Lane Estate in time for the old ladies to emerge for their swim.

Simone found that the simple things worked the best, so, dressed in her nurses uniform, with a short, dark wig liberated from the locker of a recently deceased cancer patient, she approached the glass front door as the ladies emerged.

All it took was a smile, an apology for losing her key, and the ladies let her pass. Sometimes it helped to be plain and unremarkable.

His flat was number thirty-seven, on the second floor. Each floor was a long concrete corridor, open to the air, with doors dotted along it. Simone moved confidently, passing the front window of each flat, realising that each of these windows looked on to the kitchen. In one, an old lady stood washing up; in another, she glimpsed a view of the living room through a hatch in the kitchen, where two small children sat on the carpet playing with toys.

She reached the door of number thirty-seven – the third flat before the end – with a key ready in her hand. She’d gambled on the front door having a pin tumbler lock. This was the most common type of lock, using a thin key. Duke had told her all about lock bumping. It was possible to force a pin tumbler lock with a special-shaped key with a jagged edge. The only problem was that it could be noisy. Once in the lock, they key had to be pulled out very slightly, then tapped sharply with a hammer or a blunt object. This forced up the five small pins that made up the mechanism of the lock, tricking it into thinking it was the correct key.

Duke had ordered her a bump key online, along with the suicide bags. Simone had practised with the key on her back door, but her heart was lurching as she approached the man’s door. She was pleased to see that it did have a pin tumbler lock, and she inserted the key. In her other hand was a small, smooth stone and she tapped the key sharply –

once, twice – and turned the handle.

Triumph flooded through her when the door opened. If there had been a deadbolt, this would have been near-impossible, but the door opened and she slipped silently inside. She checked for an alarm and was glad to see there was none. It seemed that the video entry-phone system had lulled this man into thinking he didn’t need extra security.

She stood for a moment with her back to the door, slowing down her breathing.

She quickly moved through the flat. The first door on the left led to the kitchen – it was tiny but modern. The hallway opened out straight ahead onto a large living room. Through a large glass window, she could see the high-rise tower of the Lloyd’s building dwarfing several other tower blocks. Inside the room there was a flat-screen television, and a large L-shaped sofa. Above the sofa, a giant photo of a naked man stared at her malevolently. An entire wall was lined with books, and the bottom shelf was devoted exclusively to alcohol: fifty bottles, perhaps more.

It was far too many bottles. Would she need to resort to using a syringe?

In the back corner was a metal spiral staircase that vanished into the ceiling. Simone climbed the staircase and saw that the layout upstairs was also small: challenging.

Her heart began to beat with excitement and anticipation. She was more excited by this than she had been with the others. She checked the location of the electricity box and phone lines, and when she was satisfied, she came back to the front door. On the wall beside the door there hung a vast array of coats: long, short, thick and thin. A small plaque was screwed to the wall, and several keys hung from it. She lifted them off one by one, trying them in the front door until she came to one that opened it.