Kerensa Moore laughed. ‘I love the idea of prank phone calls. It sounds so lovely and innocent.’

Simon scowled. Innocent? When, seconds ago, he’d described a scenario, albeit fictional, in which prank calls led to suicide? He added Moore’s tone-deaf response to his ever-growing list of evidence that so-called psychology experts knew fuck all. ‘Let’s now look at a different hypothetical situation,’ he went on. ‘Linzi and Rhian each did something wrong that Billy deems worthy of punishment by death, but they didn’t do the same thing.’

‘So Linzi drives Billy’s sister to suicide with prank phone calls, and Rhian … runs over his pet dog on her Vespa?’

‘In the case of two separate crimes, might Billy decide separate executions were appropriate – to do full justice to both? Might he kill the two halves of the pair separately because, in his mind, they’d committed two distinct offences rather than jointly colluding in a shared one?’

Why bother asking? Simon knew it was not only possible but likely. He knew Kerensa Moore wasn’t as clever as he was. This was a pointless conversation, a box-ticking exercise.

There was a long silence from Moore. Then: ‘You know what? I want to say yes. Yes! It makes absolute, perfect sense that he’d do that if everything you’ve outlined were to turn out to be the case. But then that wouldn’t work for the second pair, would it? Angela and Josh. Or for Marion Hopwood.’

The lounge door opened. Charlie mouthed ‘Food’ from the doorway.

‘Yes, it would,’ Simon told Moore. ‘It’d work for Kim Tribbeck, too, though she’s still alive. This is about pairs, just not pairs of best friends. And all three pairs work in exactly the same way. Thanks for the chat. It’s been useful. Got to go.’ Simon put down the phone.

‘That was better,’ Charlie said uncertainly. ‘You thanked her and explained you had to end the call.’

Simon was about to roll his eyes, then stopped himself as he remembered that was James Wing’s tick. He was Wing’s favourite person at the moment, even more than usual. Strangely, he found he didn’t mind so much now that real progress was being made. Simon had decided there was no point him driving to Poole when Wing was already there and would welcome the chance to be a long-distance conspirator. It had worked like a dream, and Simon had agreed to go for some beers with Wing next time they were in the same place, even though he wasn’t overly keen on beer.

‘One small suggestion,’ said Charlie. ‘Stay on the phone for three seconds longer so that whoever you’re talking to has the chance to say, “Okay, bye, see you soon” – so they don’t feel they’ve had the phone slammed down on them. But overall? Definitely a move in the right direction.’