I north London here the water is quite hard (i.e. a lot of chalk in it), so
we easily get limescale
build up in the bend of the wc bowl.

Using Harpic Limescale Remover has shifted most of it, but that seems to
have left the porcelain area under water a kind of a dirty colour. I then
used a lot more Harpic, but nothing has improved. Although it does seem to
come off a bit with lots of hard brushing. Anyone any idea if this Harpic
could be causing this?