I made myself a coffee, splashed my face with freezing water from the tap. My head felt clearer.

Here was what I knew: Charlie had said, albeit in a jokey way, that if she were going to murder someone, she would fake a drug overdose. Karen, who had never been into drugs, as far as I knew, had died from a heroin OD. Charlie had secretly visited her shortly before Karen died. Also, she had said to me, by text, that she thought Karen had taken advantage of me – the words she had used when she went to see Karen.

If I was on a jury, would I convict her on that basis? It was – what was it called? – circumstantial evidence. Charlie’s defence would be that she was only kidding about the heroin overdose, that she would never actually kill anyone.

What about her motivation?

That was simple: jealousy. Charlie hated me working for Karen, loathed me having anything to do with her. Maybe she thought I was still interested in her, that we would have an affair. But I hadn’t shown any signs that I was still into Karen, had been moaning to Charlie about how annoyed I was with her. I could picture Charlie going to see Karen on my behalf, thinking she was doing me a favour, getting my money. But why do it without telling me? And where was the cheque? She hadn’t given it to me.

I tried to think it through, how it might have happened. Charlie goes to see her, on the pretext of getting my money, and then – what? Did she always intend to kill her or was it only something that happened after she’d met Karen? Did Karen say something that enraged her, that made her flip out, her jealous fury driving her to do something terrible? She hadn’t done anything on their first meeting, so she would have had to go back.