Hi
Can you get standalone/free standing carbon monoxide alarms (ones that you dont attach to ceiling or are connected into the mains) that you could place on fireplace - they'd be battery operated I guess
Is there such a thing and where stocks or sells these
Had a look on woodies and b&q web site and didn't see anything
Can't you also use ones that normally attach to the ceiling as free standing detectors?
I doubt that not being mounted on the ceiling would adversely impact performance.
But check the manufacturer's installation instructions to be sure.