You can get limescale remover from the supermarket. It costs just a quid, and does the business - there's no other option if you live in a hard water area.
In order to stop limescale building up, rather than removing it once formed, pass the water through a water filter (eg Brita, Boot's own brand etc) before putting it in the kettle. The filter is an ion exchange system which removes the calcium compounds which cause scale.