Sorry to hear your bad news Nick. Carbon Monoxide is known as "The Silent Killer" as it is odourless and as you breathe in, it makes you sleepy. As you sleep, you breathe in more and the concentration increases. Oxygen is readily attached to the blood and released, CO holds on 400 times more strongly and blocks the Oxygen from attaching and you die.
I would never use a bivvy heater, candle system or cook inside.
If it is that cold that you think that you need external heat sources, go home, the fish ain't feeding!