You can buy limescale remover (I think it comes in tablet and liquid form) from hardware shops or supermarkets. Alternatively try filtering the water you use in the kettle by using a water filtering jug, also available in supermarkets.
We pour in about 3 or 4 glugs of vinegar and let it simmer for 20 or 30 minutes, ( You can use less time and repeat this. It all depends upon how much build-up you have).