I think there is far too much fuss made over CO alarms and they create another stream of anxiety for people .
Modern central heating boilers are room sealed and very safe as long as they are installed and maintained competently.
Further ,CO alarms with replaceable batteries are being phased out and should not be bought.
The reason why is that people continually replace the batteries and think they are safe . They press the button on the front and it beeps.
Guess what? pressing the button only proves that the sounder in the alarm works. It does not prove that it will detect CO and alarm !
To test the alarm proper, you would have to expose it to CO alarm test gas.
People replace batteries for years,not realising that the alarm is actually knackered and although it beeps, it will not detect CO and so is useless.
If you really want a COD, buy one of the new sealed unit ones with non replaceable battery . Buy it from a reputable source eg screwfix for best prices, so that you know its manufactured to British Standar and not some crap off ebay.
Fit it in accordance with the instructions in the packet which you need to read.
DO NOT fit it or stand it close to an appliance or it may tend to false trigger.