Thanks your reply - I should have added that I have tried a number of different focus types and point sizes, all with the same result. The example image I posted was one with which I tried the smallest focus point, though the others were the same too.
It does look like motion blur, particularly with the other out of focus objects in the image, however surely not at 1/680? I was also intentionally holding the camera very steadily to avoid any shake. I'm getting similar results in higher light scenes too.
Unfortunately I only have the one lens to try with, but it looks to me like it could be more of a lens issue?