‘Mr Rudolphy thanked me for returning the book. My apology seemed to be enough for him, but it wasn’t for me. That day, I made up my mind to become a policeman. And I did.’ Proust nodded vigorously. ‘You don’t see what any of this has to do with Billy and his victims. Only that Rudolphy’s is where Isobel Sturridge claims to work, and she doesn’t work there. She lied. Incidentally, I read the transcript of Sellers’ interview with her and he describes her as having very short brown hair.’

‘So you think she’s Faith Kendell?’

‘You will too when I tell you this: Elise Rudolphy’s mother, who often sat behind the till, whose creation the bookshop was … Her Christian name was Faith. I’d often hear her husband’s disembodied voice calling her. If she was busy when he called, she’d smile at the customer and ignore him until she was good and ready.’

‘But … for Isobel Sturridge to give us an alibi involving a job in a shop where she doesn’t work at all? Would she be so reckless?’

‘She has been. So, yes, she would. My foray into the case files also told me that Kim Tribbeck believes Liam Sturridge’s sister works nights.’

‘I think you’re right,’ Sam murmured. Had he known that? It rang a bell. Trouble was, when you had five murders, the paperwork mounted up; it was impossible to keep all the details in mind at once.

‘Bookshops aren’t open overnight, therefore Isobel Sturridge can’t possibly work in a bookshop,’ Proust underlined his point.

‘Sir, how did you find out she’d lied? Did you ask at Rudolphy’s?’ Sam assumed he was still a regular customer.

‘Sadly, that would have been impossible. Rudolphy’s closed down in September 2013. There are no other branches. It was a Silsford institution that travelled no further. Rudolphy’s is no more.’ Proust launched himself off the desk in the direction of his private cubicle. ‘Therefore Isobel Sturridge couldn’t have been working there in September 2014 when Linzi Birrell was murdered, or on any of Billy’s other kill dates. She gave as her alibi a shop that no longer exists.’

Sellers hadn’t wanted to risk following Sondra Halliday on Twitter under his own name in case she blocked him, so he’d called himself @howmanywomen. It sounded feminist enough to fool Halliday and it was the first three words of the driving question of Sellers’ life: how many women could he bed before he died?

He’d stopped short of using a double-bladed axe as his avatar, as Halliday and many of her Twitter associates did; there was a limit to how deranged he was prepared to pretend to be. Instead, he’d picked a cartoon of a woman who looked a bit like Miss Scarlett from the board game Cluedo, but wasn’t.