Following in case anyone's figured this out, as same happened to us!
Try sterilising in a Milton cold water steriliser - it's worked getting some food stains off spoons and bowls so hopefully works for a bottle 😊
@Kate I might have to try this! One of our blue spoons is starting to go orange 😂
@Kate second this!!! I cold Milton sterilise some of our dishes after cooking to get rid of stains x
@Kate love a bit of Milton 🤣 definitely second this!
Boiling water and fairy liquid (the real branded stuff) with a good old sponge scrub? I hand wash all bottles separately from any food stuff to try and prevent this. We also don't have the luxury of a dishwasher. Or maybe some bicarb wash?