The amount of people on here that think it's only gas appliances that leak CO is worrying...
Any incendiary device is capable. That includes bbqs, wood burners, rayburns and other cookers, water heaters, open fires, blocked chimneys, paint fumes, or the one that caught me out, the wrong cap on the chimney top which meant the dying embers from my woodburner caused CO to leak into the bedroom I shared with my newborn, through the chimney breast.
Luckily the window was broken so there was a miniscule ventilation. I dismissed my dizziness and lethargy as new mum symptoms.
I'd never even heard of CO alarms before that, but I kept dreaming about my deceased Aunt telling me to get one so I did and it went off the first night. Myself and newborn would have died in our sleep if I'd ever asked the landlord to fix that broken window..it's ironic that his negligence in not fitting the correct chimney cowl which caused the CO leak, alongside the broken window, is what saved us.
Always have a ventilation source.
And don't rely on your landlord gas safety check. I'm with a relatively reputable letting agent but the gas man they send out doesn't even check the CO monitors are working, nor does he check the gas fire (I had it independantly confirmed as 'condemned'), there's no ventilation in the room which houses the gas boiler unless I keep a window unsecured at all times (not viable, really) and even then he takes about 30 seconds to do the check.
I believe some local fire stations or councils give free alarms out.