Blue line from hot water tap on enamel bath

Well it's gone now thanks to advice from a plumber. You soak a piece of kitchen roll in water, squeeze out surplus and then douse it in white vinegar and put over the stain, ten minutes later stain gone.

Oh, nope, it seems it’s not suitable for enamel.

Cillit Bang? It seems to shine up and clean everything else in the bathroom. Though it contains chemicals and you may want something eco-friendly.

Bar keepers friend and a magic eraser.

What you could try is T-Cut original. This is a very mildly abrasive polish used to restore the colour of car bodywork. It takes a lot of elbow grease though.

Is is like a Plimsoll line? You must fill the bath any higher? ...sorry that is of no use at all but it was my first thought!