Thanks so much all for your advice and suggestions, I'm very grateful. I think I might just email the council in the first instance with some photos and a video clip to ascertain as to whether the chimney set-up and smoke etc do indeed fall under any emission regs and take it from there, as I'm loathe to start any kind of outright war, since I'm not the one living there. But it's not nice to see my mother's garden under siege either.