We bought a First Alert battery-operated alarm for £26, model FCD3N. This seems quite expensive, but we think it's worth it.
According to the box:
Carbon monoxide is one of the main causes of accidental poisining deaths worldwide. Odourless, tasteless and colourless carbon monoxide is a by-product of incomplete combustion of fossil fuels.The FirstAlert website states that CO poisoning produces symptoms similar to the Flu, so you may not be aware of the danger at first. You should install at least one carbon monoxide alarm in your home, installed near the sleeping area and with an audible warning signal. Do not have one in your kitchen; instead, place it in an adjoining hallway or room.
There are many features a carbon monoxide detector can have, but the most important is where it gets its power from. Battery-operated CO alarms are the most common, and like a smoke alarm will warn you when the battery needs replacing. You can get mains-powered detectors that plug straighr into a socket on the wall, and can have digital displays showing the actual CO reading. It is also possible to program some models so they tell you which room they're in when they are sounding the alarm - useful if you have a few of them!