I’m looking for the title of a dystopian middle grade or YA book I read in the late 1970s or early 1980s. An orphaned girl (tween or early teens) and her younger brother live with their aunt and uncle in an apartment building in a big city. (Maybe in the UK? I have a vague feeling that some things didn’t seem familiar to this midwestern suburban kid.) The uncle is mean and doesn’t like having them there. There’s a disaster of some sort and the uncle flees with his family, leaving the niece and nephew to fend for themselves. There’s no power or water and when they run low on food they leave the apartment. The girl fills a canteen or bottle with water from the toilet tank, saying she was grateful that at least her uncle wouldn’t let her aunt use the cleaning pucks that turn the toilet water blue. Later there’s some sort of charismatic leader and evil government. I don’t even remember if it was a particularly good book. It just occurs to me sometimes and I’d like to remember the rest of the story.