How to defeat these stains

[WasteOfPaint]

Over time, all our bedding had developed yellow stains. After some trial and error I got most of the items back to white using Borax. Except the duvet cover pictured, where the stains are actually more pinky-brown than yellow. Borax didn't work on this and neither did Vanish/Napisan. I assume the stains are different in nature to the other ones which I successfully removed. Any idea what I could try on this, or do I just consign it to the bin?

Try leaving it out in the sun for a day or so after another wash. I got rid of several yellowy stains that way.
If that doesn't work then you may have to write it off.

Do you have very soft water? We do, and we do get browning on things a bit, I think it's peat.

Thanks, it's already hung in a sunny conservatory and hasn't really helped.

Nope, very hard water!