So I've phoned the Gas Emergency Line (0800 111 999) and now I have to wait for an engineer to show up. And I have a headache, blocked nose, ears and sinuses from the smell, whatever it is. And a weird taste in my mouth. I don't think it's warm enough to take the laptop outside, somehow. (It isn't plugged in.)
In a way, I'm hoping it is a gas leak, because at least that's fixable. If it's some other kind of organic solvent randomly attacking the house, I don't know how we'd find it to stop it.
Update: Gas engineer has been. Gas smell has also gone. Mystery! I feel exhausted, nauseated and crappy though, so I'm going to bed as soon as Richard's home.