Spraying air freshener. The Asbestos Palace reeked of diabolically eggy bottoms when I came back from the pool. I assumed something awry with the drains so spent a while tipping bleach down them, but the smell lingered undispersed in the hallway. Tentatively sniffed the dishwasher, washing machine, downstairs loo – seemed fine. What the hell could it be?
Ah, what's located under the stairs...
Anyway, thanks Bad Cat.