A. You can buy some tablets to put in your toilet that should do the trick, such as Harpic Power Plus tablets. For heavy limescale build up, drop two tablets into the bowl of the toilet and leave overnight.
In the morning, scrub with a toilet brush and flush. You can also buy toilet cleaning products with limescale remover in them, such as Domestos Zero Limescale, which should prevent limescale building up with regular use.