If you mean in the toilet bowl, first, use something to remove enough water from the bowl so the limescale ring is out of water. Then, try your mixture, but apply it a number of times, perhaps even soaking a cloth in the mixture and laying it on the ring. If this doesn't help, then, at least here in the U.S., a product called CLR or Lime Away works very well. But the secret is to not dilute it with the water in the bowl... Good luck! (Personally, I'd use great caution with hydrochloric acid)