Yellow stains on window frames

[werth]

Hi
These yellow stains have appeared on my window frames recently. Please advise how can I get them off? I have tried HG uPVC cleaner, sugar soap and white spirit but no luck.
Also, if I can't get them off, these windows are less than a year old, would this normally be covered by the warranty?

 

[Wayners]

uPVC solvent cleaner.
Go easy. Don't splash it on. Just put some on a microfibre cloth and rub gently.
It's very solvent and can damage if you go mad with it and rub hard for ages.
Also not nice to skin