Simon stood up, walked over to the window and looked out at the blocks of flats opposite. He found it easier to speak in a group situation when he wasn’t looking at the people he was talking to. ‘Angela and her husband Russell owned a four-storey townhouse in Chiswick with a large outbuilding in their back garden, from which they ran their business: a small kung fu school. That’s a martial art. They didn’t make a fortune from it, but they didn’t need to. Both Angela and Russell had inherited an eye-watering amount of family money, so the school was more of a hobby than a necessity.’

‘Not fair, that,’ said Grace Woolston. ‘It was a proper business: professionally run and the classes were popular. Just because the McCabes had money doesn’t make them dilettantes.’

‘All right, fine,’ said Simon. ‘Crime scene: kitchen of the McCabe home. Same as the previous two victims: cups with remains of drinks found – one tea for Billy, coffee for Angela. Angela, like Rhian Douglas, was shot in her kitchen, but – key difference – Angela was the only one of the four not to be shot in the back of the head. She was shot here.’ Simon pointed to the spot between his eyebrows. ‘By her body, on the floor, was a biscuit tin and biscuits scattered everywhere. Theory: Billy was preparing to shoot her in the back of the head while she was facing the kitchen worktop, mugs, kettle, etcetera, but then she turned round, biscuit tin in hand, to offer him a biscuit – so he shot her face to face.’

‘I think Billy’s conflict-averse,’ said Kerensa Moore. ‘He likes – maybe even needs – everything to be friendly.’

‘Then he ought to consider not murdering people,’ said Proust.

‘I’m not saying he’s a great guy who should be let off,’ Moore clarified. ‘Only that his preferred M.O. – cups of tea first, then shot to the back of the head while unobserved – suggests he might be uncomfortable with overt hostility. He wants people to like him – right up until the moment he shoots them dead.’

‘That’s … not a given,’ Simon changed course to end his sentence more tactfully than he’d first intended. ‘He might just be efficient, or risk-averse. Why allow your target to see you produce a gun and aim it at them when you can wait till their back’s turned and surprise them? If they’ve got even a split second’s advance warning, they might duck out of the way and then you’ve lost control of your kill.’

‘Wow. Spoken like a true scary murderer!’ Kerensa Moore laughed.

‘How do you think I catch these people year after year?’

‘Fair point, DC Waterhouse. You’re right: your analysis of Billy’s character is as likely as mine to be correct. It’s good to air as many as we can come up with. There might be other equally plausible possibilities. Let’s hear as many as we can.’