Well, it went off once, saying the carbon monoxide level was 999ppm which is way too high. So I reset it, and it went off again. So I've had to turn the gas off at the mains, which means no heating, open the window (excellent, it's well below freezing) and the gas engineer will be here within 2 hours. And while I was on the phone to Transco, it went off again.
I wonder if this is why I've been feeling fluey, headachey and queasy this week. I thought I was just coming down with something. I do have the boiler serviced annually, so I am not happy.