I think the injury was caused by the saw, which nicked Ifield's neck, hitting an artery. That's why the injury was small, but it would have caused him to bleed out. There would have been heckova lot of blood, including Thandie's.
I think there must have been more than one person in the flat. How did Thandie get the wrist marks otherwise? Like she'd been tied up?
My money's on the husband being involved. I think it was his car, stalking Thandie, pulling up at the flat. I think he waited for Thandie to come out, and when she didn't, got himself in. Not sure how. He did something in there, either to protect Thandie, or Thandie may have covered for him.
Ifield died as a result. Thandie cleaned up, but was left with poor memory/hearing as a result of the injury. That's why she heard "30" instead of "13". She couldn't have gone to hospital with the injury, so I reckon hubby helped her clean up. It must have been a thorough job if the only blood left was the splatter.
Another thing: how come Arnott called Hastings to the scene? He's not murder squad.