I cleaned the shower with vinegar, just to see what happens, and it got sparkling clean. Now I'm ashamed. I thought as soon as you can see any limescale at all you need, I don't know, emergency measures... something that comes in a brightly coloured spray bottle and plays a fanfare when you pull the trigger. I thought vinegar was just good for cleaning burned pans.
I wonder what other stupid things I do, maybe I'll take a day off some time just to have a good think about that.