Yellow Staining-Synthetic(not-metal) bathroom bath

I think these baths are polycarbonate, if that makes any sense to anyone. It's not that old about 7 years, and more uesd as a shower. This yellow staining is a bit like a "water-mark" as in money watermark. That is that you have to look at a specific angle to see it. It's not just in the bath, but also on the ledges, horizontal surrounds. have tried to remove with Cream-cleaner, and also with toilet cleaner, thinking it might be something in the water. Anyway all of these made no impression. Anybody removed similar stains.
thanks

Domestos or Parozone are stronger than cream cleaners but make sure you use rubber gloves.

When what I call hard water marks appear on our bath I use a Brillo pad so one of these might be worth a try.

Many years ago in London we had a similar problem when we moved house, my neighbour asked her dad who was a plumber what I could clean it with and he said "spirits of salts" which I bought in a local hardware shop but maybe such things are not available any more