Thanks for your reply Ben.
I'm seeing it directly in my render view.
Heres a quick image of the test cube I setup. The image on the left is the rgb and the one on the left is its alpha.
http://img116.imageshack.us/img116/1381/motionblur1nm.jpg
As you can see its alpha looks fine so why would I be getting this dark black edging?
Any help really appreciated.