LImescale is calcium carbonate that has built up inside your kettle. It is a hard mineralised substance that can only be removed by using something acidic. Lemon juice or ordinary vinegar work well. Mix enough (half chosen acid and half water) to cover the area affected, and leave for at least an hour. Hard deposits may require longer, even overnight. Pickling vinegar is even stronger than normal and will be more effective on really stubborn build-ups. Lime juice is also stronger than lemon juice. You will need to rinse out afterwards to get rid of all the acid. The smell of vinegar will eventually go, but the lemon or lime will leave a much more pleasant smell!