Limelite active gel or Viakal are great for cleaning and delimescaling sinks and showers, but the best for doing the same in the loo is domestos zero limescale..squirt it round the bowel, leave for 30 minutes and then use the loo brush to give it a bit of a scrub off.
These types of products are especially made for limescale removal and really do work much better, and far easier, than ordinary cleaners, disinfectants or bleaches particularly in hard water areas.