I stood on the landing, trying to work it out. What was Ben looking for? Perhaps he’d been disturbed. Perhaps he had still been here until a few minutes ago, and had bolted when he heard my car. Or perhaps he hadn’t gone at all.

Perhaps he was still in the house.

Gripping the rolling pin tighter, my heart thumping, I stood very still and listened for any noise, any creak of the floorboards that might give him away. Silence. Behind the silence, in the distance, the ever-present dull hum of traffic on the North Circular. My house suddenly spooked me. Ben. Just in case he was hiding I went from room to room, quickly pulling open wardrobes, looking under beds and behind doors. A full sweep of the upstairs, followed by the same in every downstairs room.

There was no one. I finally began to relax and went back into the kitchen to return the rolling pin. That was when I saw it on the kitchen counter, next to the kettle.

A shotgun cartridge.

It was pink, standing upright, with a brass base and lettering up the side. Eley Hi-Power #00 Large Game. It was thick and heavy and I wondered suddenly what it would feel like to be on the receiving end of the lead shot inside.

I didn’t own a shotgun, and only knew one person who did.

There was a note underneath the cartridge, A4 letterheaded paper from Ben’s company, Zero One Zero. He had once told me what his company name meant, something arcane and techie to do with programming language or binary code or something similar. I couldn’t remember exactly. On the paper were just five words, written with a thick black marker.

STAY AWAY FROM MY WIFE.

I stared at the paper for a minute, my pulse thrumming in my ears. Until now this had felt like some stupid game where I didn’t quite get the rules, but for the first time I was genuinely worried, frightened. What if William had been here? What if Ben came back tonight, in the middle of the night? What do you do if you wake up and a man is standing over you with a shotgun? It didn’t bear thinking about. But the fact was that he was out there, armed and dangerous, and able to break into my house.

I called the police and reported the break-in. The operator spent five minutes going through my personal details, then asked me to describe what I thought had been stolen.

‘Nothing’s been taken. At least not that I’ve found so far.’

‘There are no items missing from the property?’ she said, a sceptical tone in her voice.

‘Don’t think so. But he left a shotgun cartridge.’

‘Beg your pardon, sir?’

‘A shotgun cartridge and a note, a warning, were left in my kitchen.’