So as mentioned above, I finally got my binder!
Already, it's miles better than my Les Love Boat binder and I'm pretty overjoyed with it. It turns out I did measure for a small and that's what I went with, in black if that makes any difference.
From the front, I am pretty much completely flat. From the side, there is still a little "something" but I think it looks much more natural and like a male chest, especially compared to my old binder.
I'm really satisfied with it so far, although it is a pain to get on and off it's quite worth it!
Here's a picture for anyone that's interested:

This is me in a fairly fitter shirt, so you can see it works pretty well.