Is the vignetting intentional or is it a function of the camera? I don't think it necessarily detrects from the image itself, and in some ways helps to keep the focus on the subject. Have you tried increasing/decreasing the amount of vignetting to see the effect on the picture?
Thanks for the kind comments everyone.
Re the vignetting. The picture was taken with an IR and an ND8 filter - unfortunately this does cause vignetting a little.
Usually I would crop the picture but I kind of liked the effect it gave so left the picture untouched.