‘I believe you, Isobel,’ says Simon. ‘I think you acted alone, with a motive that would have crumbled if you’d had to discuss it with anyone else. It obsessed you, but, at the same time, you knew the logic of it was too fragile to prove itself outside of your head. That’s why telling the truth would have ruined it for you. Even telling your own brother.’

Isobel’s staring at me. ‘You were going to be the first,’ she says. ‘But on the night I was supposed to give you your book, you were there with my brother. I wasn’t expecting that. Liam saw me trying to put the book in your bag. My own silly fault! I chose to come and see you in Canterbury of all places because Liam sometimes worked there. It seemed almost meant to be. When I saw that Liam had spotted me, I ran away. Hid outside.’ She turns to him. ‘I saw the two of you kissing on the street, and you put your hand up her skirt.’

I get a grimace from Pencil-up-the-arse Dunning and a half-smile from DC Sideburns that looks like a secret message of approval.

That’s right: I have skirts. I let men put their hands up them sometimes.

‘I love my brother – he’s the only person I have left – so I knew I had to change my plan,’ Isobel goes on. ‘I couldn’t kill the woman he loved.’

‘“Loved” is putting it a bit strongly,’ says Liam.

‘So, I adapted,’ the killer called Billy Dead Mates says in the voice of an educated woman in her early forties. ‘I decided I’d leave Kim till last. There were always going to be six deaths. It had to be six.’

‘Why?’ Simon asks.

‘I knew of six cases, and only six, who qualified,’ Isobel says with an earnest expression.

‘Victims,’ says Dunning from the sidelines. ‘Victims, not cases. You killed real people, with families who loved them.’

‘Do you know anything about numerology?’ she asks him. ‘I didn’t used to either. Lane has a book about it. The number 6 is the most harmonious of the single-digit numbers. It’s also about sacrifice, and … that’s what it was, for me, what I did. Too much harm had been done that couldn’t be allowed to stand. A symbolic protest was necessary, or else how would the world ever know that it mattered to anybody – enough to take extreme action?’

Simon nods, encouraging her to go on.

‘I decided to wait, see if Kim and Liam broke up. I could kill her then, I thought – later.’

Sure thing, Freakoid. Happy to fit into your schedule wherever’s convenient. Let’s diarise.