I've just finished watching this and as much as I dislike Sion Jenkins, I just can't see how he would have killed Billie-Jo.
If it wasn't a premeditated crime, then somehow he enters the house, finds a weapon, bludgeons her to death - brutally - blood everywhere - yet gets less than one whole spot of blood on him (apparently the micro spots when put together still equal less than a spot of blood if you'd cut your finger or similar) all in a three minute window? And he then returns to the car, not out of breath, not red in the face, not shaking, and perfectly calm and normal?
Looking at the crime scene there was blood everywhere - all over the French doors, all over the patio, yet it goes nowhere near the attacker? I just can't see that that would be possible.
And the plastic stuck up her nose - if Sion killed her in a violent rage, why then stick random plastic up her nostril at the end of the attack? He wouldn't have been aware of the plastic obsession of Mr B.
Sion Jenkins does certainly appear to be a narcissistic prick, but I can't see that he killed poor Billie-Jo.