More and more I am of the opinion Chris Dawson killed Lynne accidentally - probably in a fit of rage. I think he has probably lashed out at her, like he had so many times before and for whatever reason things have ended very badly. Perhaps the 'lovely drink' he had just made her was a factor?
Anyway, I think as a result of Chris lashing out, Lynne has fallen and banged her head on the floor or against a solid structure, and it has proven fatal.
I know there are many things pointing to a more premeditated act, and I am in no way sympathizing with Chris, because I think his next act was probably to call Paul and, either receive advise about, or perhaps assistance in, disposing of Lynne's body.
Does anyone else see the above scenario as likely, plausible?
Nope, there wasn't a hope in hell that Chris accidentally injured Lyn. That would mean Joanne had no place to return to and the thought of Joanne enjoying herself with a group of people her own age was too much for him to bare. He urgently needed a sitter as well.