That's interesting, as the vitreous guys speak most disparagingly of bleach - and bubble bath, soap, hair dye, soap powder, and in fact left me feeling I should just look at the bath, and never, ever use it. (Put the coal in it? - Oh no, more iron stain, from the pyrite.) Vinegar, and lemon juice outlawed. There are products in America and Australia, but people who import related products haven't brought in the rust stain versions. Ammonia wasn't mentioned, though, and CaCO3 is easy enough to get round here. So thanks - I might give that a try. But before polishing that bit.

Penny