Toilet in new flat was thick with limescale. I tried loads of descalers which did nothing, vinegar and bicarbonate of soda which shifted a bit but not much, I wore out 3 pumice stones, coke just stained it dark brown 
.
This morning I emptied out the bowl and squirted a bottle of lemon juice directly onto the limescale which I left for an hour, then filled up with hot water and 2 more bottles of lemon juice and half a box of bicarb and left for another hour. I then gently prized with a plastic picnic knife and gues what? The whole lot just pinged off. In great big chunks. I’m so pleased, it’s like a new toilet.