That's limescale build up, you need acid to shift it, not alkali.
e.g. http://mistralie.co.uk/catalogue/product/97/Acidic-Toilet-Cleaner---Heavy-Duty---Premium-LOOBR
@gbj_tester started
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That's limescale build up, you need acid to shift it, not alkali.
e.g. http://mistralie.co.uk/catalogue/product/97/Acidic-Toilet-Cleaner---Heavy-Duty---Premium-LOOBR