‘She does, though,’ Simon cut in. ‘Faith Kendell was the maiden name of her maternal grandmother, Faith Rudolphy. She knows that. And now she knows we know it too. This late in the day, denial’s not your best option, Isobel. We know you were at the RGI the day Marion Hopwood was murdered. We know you lied when we asked you where you were on the days of the other four murders: you said you were at work at Rudolphy’s, the bookshop. You weren’t. There is no Rudolphy’s any more. You used to work there, but then the shop closed down – more than a year ago, in September 2013. At the time of the first four Billy Dead Mates murders, you worked nights at Rawndesley Services on the A1. You still do. Which means you have afternoons free – to sleep, usually. Except I don’t think you slept on those four days. Did you?’

Isobel made no attempt to speak, though her mouth was working. With her bottom teeth, she kept trapping her top lip as if trying to bite the tears as they fell.

Simon wondered how and why someone who’d worked for her family’s bookshop had ended up working nights at a motorway service station. Why not look for another job in the bookselling trade, or train to be a librarian?

Perhaps it wasn’t so jarring a lifestyle change; it was simply that Simon could sort of imagine himself working in a bookshop – Word on the Street in Spilling, for example, where he bought all his books – whereas he’d regard serving sausages to refugees from the M1 in the middle of the night as a form of torture.

‘I’m wondering if we made a mistake referring to our unknown killer as Billy for so long,’ he said to Sam. ‘Must have made it easier for the real killer – Isobel here – to imagine her crimes were being committed by someone else. Her name’s not Billy, so it can’t be her. And she’s certainly not a man, so it really can’t be her. We must be looking for someone altogether different.’

‘Are we, Isobel? Look, if we’ve got something badly wrong here, tell us. Please. If you’re scared of somebody, if you’ve been threatened …’ Sam ran out of imaginary afflictions. ‘We can help you.’

‘And if you’re Billy, we can also help you,’ said Simon. ‘We’ve got enough to charge you already. Any jury’d convict. The way you’re treated from here on in, the experience you have as a convicted criminal, that can go one of two ways. If you want it to go the better way, start helping us. Then we’ll help you.’

A noise from her at last.

Simon’s heart did a jump-start. ‘Pardon?’

‘I’ve tried to help you.’

‘How?’

Silence.