Carbon monoxide poisoning is something I take no chances to play with. However, most of the cases result from the Room heating appliances that use Gas without automatic cut offs. It is best to have an exhaust fan mounted near a window in the kitchen to keep the air flowing in and out. And it is best to avoid the heating appliances that run on fuel within the bedroom. Other than that it takes around one to two hours in a medium sized room with the heater running on fuel and all windows and doors closed, to make a person faint due to CO poisoning, especially when the room is usually kept poorly ventilated. Anyways be Cool and stay safe!
Jemma116 is correct in that you need to have a detector in every room and check the batteries at least once a month. I would like to add that some devices are built with automatic shut off features to help prevent carbon monoxide poisoning from happening. One such device is a gas hot water heater. Please do not forget to take care of your smoke detectors as well.
The simple thing to do is to move the carbon monoxide monitor to the kitchen, or buy another to put in the kitchen. You should have a monitor in every room and also get it checked periodically to ensure that it is still working and the batteries not flat.
The gas flame should be blue.
Don't forget to check the pilot light on the central heating and ensure that it has not gone out and that any flues and chimneys are swept and kept clear to allow the fumes to escape out. Check any other electrical appliances - your toaster so that it does not have burned food. It is not just gas appliances that need checking - if you have an open fire, you need to ensure the chimney is clear too.
Check out the web link below that gives good advice and information with regard to carbon monoxide.
http://www.gassaferegister.co.uk/learn/carbon_monoxide_kills.aspx
Ensure there is adequate ventilation.
I experienced carbon monoxide poisoning and felt very unwell for a long time. In my case it was a gas fridge on a boat that had the outlet flu blocked. Since that time, I have been told, the boating holiday companies (in the UK) use the electric power from the engine to work the fridge.