torge conrad maguar wrote:
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?

Use a pumice stone.

It works and alot cheaper than the chemicals. :-)