‘The blonde woman wasn’t looking at Muriel,’ said Woolston. ‘Instead, she was looking straight ahead, but she was smiling, as if she’d just spotted a friend, when in fact she was staring at the far wall of Muriel’s lounge. Understandably, Muriel was unsettled by this – even more unsettled when, instead of stepping back from the window, the woman moved to the side in a way that looked – and I quote – “funny and just not right”. She was heading for the front door, Muriel assumed, but when Muriel went to look there was no one there. The blonde woman is – or I suppose I should say “was”, from DC Wing’s hints – a person of interest.’

‘Was,’ Wing agreed. He sat forward and rested his head in his hands briefly. ‘I’ll start with the geography, because it matters. Rhian Douglas and her boyfriend Dolan lived in the first of a row of four tiny terraced cottages, numbers 17 to 20 Nettle Edge, which isn’t a proper street. It’s unmade: a mess of bumpy potholes and about as narrow as it’s possible for a road to be – only a few centimetres wider than one car or van. No pavements. When I stand in Rhian and Dolan’s living room with my car parked outside, I can’t see the wheels.’

‘Why would you want to see the wheels of your own car while interviewing a witness?’ Proust asked.

‘I didn’t say I would or did, did I?’ Turning pointedly away from the Snowman, Wing said, ‘As you all know, we assigned our mysterious blonde at the window to a sketch artist. Before we had a chance to do anything with his handiwork, we got a call: Muriel Pearson’s son. Asked if I’d go round and see his mother again. She had something she needed to discuss, he said. She was worried about it, and scared … As you can imagine, that was enough to get me haring round there. At first the old lady was all het up, apologetic, she never meant to tell a lie, all that. Since we all know people who say that usually did mean to lie, I thought, “Oh, here we go”, made the right noises and sat back, waiting for the confession.’ Wing laughed and shook his head.

‘And?’ said Simon.

‘Muriel Pearson didn’t lie. But she did make a mistake. A fu— An important mistake, the correcting of which leaves us with no more mysterious blonde woman to search for. She was never there.’

‘The blonde woman was never there?’ said Sam.

‘Nope.’ Wing smiled enigmatically. ‘No grinning blonde lady outside the window who moved to the side in a peculiar way. It didn’t happen.’

‘So how did a woman with decent eyesight, who didn’t mean to lie …? I don’t get it,’ Grace Woolston said.