Answer #1: Anything acid, for example
flat coke or most sodas, vinegar or lemon juice -- Davina Martin Comment
Answer #2: Of course the stuff that has been produced to descale kettles is very effective and better than the ones I mentioned earlier but they can be very effective. I now use water filters since I found out that our local water is not so good and find that there is no build-up of limescale; the inside of our kettle is almost as shiny as the day we bought it. -- Davina Martin Comment
Answer #3: Vinegar left overnite, you can buy it quite cheaply in quantity. Use to clean surfaces as well, a cupful along with the wash works to keep colours fast and cleans and descales the machine - Cheers -- Leslie McDonald Comment