‘She sits in her chair all day long watching telly she can’t hear, barely taking her eyes off the screen,’ said James Wing. ‘Maybe the day Rhian was murdered was the first time the van driver parked in that particular spot, right outside Muriel’s window. In any case it was the first time Muriel saw or noticed her.’

‘You asked her that?’ said Simon.

‘Yes.’

‘I never thought the blonde woman was Billy,’ said Proust. ‘If you’ve just shot someone, you don’t wander along the street and smile in at the neighbours before disappearing – you scarper.’

‘Simon, what are you thinking?’ Sam asked.

‘He’s wondering when to launch his brilliant new theory,’ said the Snowman. ‘The Asda lady – the real woman in the photo – secretly follows the vans that bear her image, and murders the people they deliver shopping to – except in this case she miscalculated and got an innocent neighbour by mistake.’

‘There’s nothing implausible about a real person being there that day as well as the picture on the van,’ said Simon. ‘We know someone else was in the vicinity: Rhian Douglas’s killer. Who might, for all we know, be blonde and smiley, with a fringe – and for all Muriel Pearson knows, if she’d never seen that van before and sits in her chair all day, never looking out of the window. How does she know?’

‘She positively IDd the Asda van picture as the woman she saw at the window,’ Wing said. ‘When I showed her the photo, she turned pale and started to cry. “That’s definitely her,” she said.’

‘Your ignorance of the female anatomy is, as ever, your Achilles heel, Waterhouse. Real women don’t slide to the side as if on wheels. They walk. On these things they have called legs.’

Unbelievable. They’d all missed it.

Simon smiled and stared at the wall, like a picture of a man on the side of a van.

‘Do you really believe all this?’ DC Colin Sellers held up page 27 of Lifeworld magazine so that the woman opposite him could see her own words. Would she turn away in embarrassment when confronted by her own stupidity?

The two of them were sitting at a window table in the Artesian bar of The Langham hotel in London; her choice of venue, not his. While he waited for an answer to his question, Sellers silently listed all Sondra Halliday’s worst physical features – too-small eyes, too-small forehead, bitten nails – in a futile attempt to persuade himself he didn’t fancy her.

He did.