For Removal of Limescale caused by hard water
Firstly do not use Bleach,Bleach won't shift limescale, it'll just remove any staining so the limescale is less obvious.
To remove limescale you need something with an acid base - Viakal or Limelight if you want brand names, or vinegar (distilled malt does the job quite well). Adding heat speeds up the reaction, just make sure you don't splash as you go. Removing water from the bowl before you start will make whatever goes in more concentrated and so more effective.
Most branded toilet cleaners that specify they remove limescale (NOT toilet bleach) have an acid base and will most likely be more strongly acidic than over-the-counter acid - if nothing else is working it might be worth trying an entire bottle of a limescale removing toilet cleaner (such as these by Parazone, Toilet Duck and Harpic) and leaving it overnight.
One of the Best and Most effective, that does what it states and is very "Highly Recommended, works fantastic every time is Harpic Power Plus", it can be purchased from most supermarkets.
http://www.harpic.co.uk/products/deep-cleaning/harpic-power-plus-liquid/