That's it. I'm going to get a job with Scottish Gas. The Gas Man came, as promised, and confirmed my (MY) diagnosis of the problem. A long flue, a very cold night, cold air rushes heavily down the flue, swamps feeble heat of fire left on "low" to warm the room where we were not sitting, fumes can't make it to the top of the flue and instead flop into the room. Result - screaming carbon monoxide detectors.
So we've got our fire back. And we've just heard that the cost of gas is rising - by 50%? Before we know it, we'll be huddling over an open fire burning scraps of wood and potato peelings. Oh - and maybe a coal briquette. We burned them, apparently, when I was two and there was a shortage of proper coal. But then there had been a war. Oops - are we in the aftermath of another one?