Firstly, v nice render.
Second, there still is a little confusion over what helps, but I've found that, in the Advanced tab of the renderer setting the Ray detail region to "in camera" helps. EDIT - according to Matt, not really - see this answer
http://forums.planetside.co.uk/index.php?topic=7019.msg74707#msg74707 Increasing GI blur may also help, but apparently at the expense of detail; and increasing GI samples also helps.
One other thing that sometimes helps is exporting different crops as EXR, and then exposing them slightly differently, e.g. 0 and 0.1.
Finally, a separate GI pass has been requested a number of times - I believe it's not straightforward.