Long story short:
I arranged with Small World's DH to surprise her with a birthday cake at inventions last night to "get back at her" so to speak for all the surprises she'd given her daughter this week - to give her a surprise of her own.
Small World was told it was my idea when she got the cake so she wants to kill me for it - it was done in Inventions so she got characters etc.
I mentioned to Small World that I'd let Diane in on the secret to cheer her up yesterday and that Diane was amused (like I was), so she wants to kill Diane as well.
Think that's the basic plot here