[Sundae]

I'm going to try the bicarb paste first.
The trouble with the Coke is that these marks are above the waterline, so they will simply get a cola shower as opposed to soaking in it.

One of the things which we can't puzzle out is that she uses limescale remover already. And none of the marks are raised. Or notably indented on the flip side - neither are they in places that get particular scrubbing. Argh.

Toilets and plumbing are expensive. Approx £200+ (parts & labour).
Of course when it comes down to it they will have to work it out between them. I know how important it is to her. I also know Dad will refuse to spend the money while they still have a functioning WC.

Thanks for the responses.
I will get back to you.