‘Also, you don’t know for certain that the two pairs had no one in common,’ said Kerensa Moore.

‘We pretty much do,’ Simon told her. ‘Unless all four murder victims were keeping their friendship with Billy secret from everyone they knew. These weren’t secretive people, though.’

‘Well …’ Moore swallowed visibly. ‘I haven’t been invited to start, officially, but …’

Proust waved his hand at her in a batting-away gesture – one that would say quite unambiguously to an untrained observer, ‘Get out of my sight.’

‘He means start,’ Simon told Moore.

‘All right, then. Given that we’ve reached this point in the discussion naturally … I think we have to consider the possibility that Billy was, secretly, a friend of each of the four victims. Extensive interviews with all four families have turned up no one who knew them all, yet we believe that each of them – Joshua, Angela, Rhian and Linzi – invited Billy into their home, and that three of the four made him at least one cup of tea … Who would you welcome into your home and make tea for, when you’ve not mentioned to your nearest and dearest that you’re expecting a visitor?’

‘A canvasser, a charity worker, a market researcher doing a survey,’ Simon reeled off the first three possibilities that occurred to him.

‘Maybe, yes.’ Kerensa Moore rewarded him with a smile. ‘Or maybe a secret friend who persuades normally open people to enter into a conspiracy of silence with him. Either way, this killer is skilled at manipulation. Four murders, unsolved, and no leads apart from a sighting of a blonde woman at one crime scene who is as yet unidentified.’

‘Our blonde woman’s gone up in smoke, I’m afraid,’ said James Wing. ‘But I can think of one possible lead no one’s mentioned. You’re all going to hate it.’

‘Let Dr Moore say her piece.’ Proust glared at Wing. ‘I’m not ready to hear another of your opinions. It hasn’t been long enough since the last one.’

What possible lead did Wing have in mind? Why couldn’t Simon think of it?

‘I don’t think it’ll surprise any of you to hear that I think Billy has issues around friendship,’ Kerensa Moore went on. ‘The obvious conclusion, I suppose, would be that he or she never had a best friend and always yearned for one. But somehow … I don’t know. I think I’d expect someone with that motivation to kill, for example, several of their former classmates, or a group of friends on a fun night out together. Not one pair, then another – and more pairs in the future for all we know.’

‘Are you making this up as you go along, Dr Moore?’ asked the Snowman. ‘I swapped a sizeable chunk of budget for all you know, and so far it seems to be not a lot.’