I'm new here, trying to help someone else in turn for asking my own questions!
You can get UPVC cleaner from DIY shops and big supermarkets, which seems to get most things off, including ingrained stuff. We've got some pretty serious stains off with it before, so it's possible it would work on rust. The trick is to leave it to soak in for a while. Might take a couple of applications as well.
Hope that's of some help!