Well, on Tuesday I came to realise what Mum had known for ages: that there was a different, distinct but vague smell of gas coming from behind the sofa in the living room. After mentioning this for the fifth time I finally decided to locate the source, where the smell was much stronger. More opening of windows, etc, but this time we called the gas man in.
The gas man arrived about two hours ago. I switched off the gas supply and explained the situation, and managed to point him to the specific floorboard he needed to lift up to get to the pipe. He immediately discovered the problem: the section of pipe connecting the thick and thin pipes at either end wasn't properly soldered onto the thin end. It could apparently have easily dislodged completely, pissing out gas at a much increased rate. Comforting thought.
Fortunately we just wanted the pipe capped anyway, as we no longer needed it since we had the central heating installed 21 years ago, before I was born. We also wanted the 27 year-old fire taken out completely, as since it had originally been installed by cowboys it was a fire hazard in itself.
What gas man and I discovered was the section of pipe leading to the fire from the now capped joint was free-standing, which was disturbing in itself. Still, we managed to get that fire out in no time, and the pipe is now in the way but harmless.
Unfortunately, I am now covered in dust from the clean-up operation that followed, clearing out 27 years' worth of dust and rubble which fell down the chimney at some point. Gah.