So here's an update. We initially contacted our flat manager. However, they don't manage that flat, so they have no pull. They're very sympathetic but they seem to only be able to advise.
We then, following advice here and from them, tried making contact via a polite note, which I took a photo of. We left it in their slot yesterday after I opened the door yesterday morning and found that a creature of some type (or maybe the wind, but it seems unlikely) knocked over the bag and a number of crisp bags and things (like, the plastic top to an instant food container that had contained some kind of sauce) were scattered on the pathway. I also took a photo of that.
Today the note is crumpled up on the top of the trash bag that is STILL THERE. The girl picked up the trash that had scattered but instead of bringing it to the rubbish area she just put it in another plastic bag and left it there. These people are obviously not considerate or open to reasonable communication (I promise my husband and I wrote the note together and it was not inflammatory at all, and was very brief).
I visited our flat manager at the office on my way home and he said that considering that, his advice is to go to the council. So that's what I'm going to have to do, I'm afraid. I had hoped that knowing this is unsafe/inconvenient would be enough for them to stop but obviously the note being crumpled up in plain sight is a big EFF YOU to us (especially considering the fact that whoever did it, did it before going out PAST the rubbish area). I really hadn't wanted to go 'nuclear' but if this is what our manager says to do, I'm going to do it.