I searched the net and found that it's best at knee height as your sleeping position is on average knee height and is more likely to alert you when you are asleep early, than having it higher. CO2 mixes with air not rising to ceiling like smoke does.
you can also contact calor gas www.calor.co.uk they will know. See top of red band to right in dark area is contact calor click then look for ask a question send same to them using their online form.
bob
June 2012