Thanks for your replies everyone, I really appreciate it. :)
I went out again today so she could show me how to put together their feed. I also asked her if she would be comfortable with me bringing a lead rope in there with me for the first little while. I said it's just because I don't know the horses very well, and she seemed to be fine with it.
The mare ('Gem') would not stop moving her bum into us! Last time it was just once, but as soon as she finished her feed this time she did it over and over again while we were talking.
But in saying that, I don't think they're aggressive at all; just kinda oblivious to respecting your space. They are pretty much spoilt, well loved pets. Which is absolutely fine! The only reason I'm so strict with my own two is because if/when my bossy old mare wants her paddock mate out of the way, she wants it done fast. Which is potentially dangerous at feeding time if they don't already know I'm not to be shoved around!