My own mother swears by this trick. As we know all kettles no matter how expensive with daily use will start building up with limescale.
The trick is, half a cup on vinegar in a kettle topped up with water over night. In the morning boil your kettle and pour this water down the sink. You will be amazed at what flakes come out with the water/ vinegar mix. Fill it with water, reboil and empty to wash away any extra vinegar
PS, don’t forget about your vinegar and water mix because it does not taste great in a coffee!