Thanks for all the great comments.
I was worried about cropping out the sky as that makes the wooden post bisecting the image much more noticeable.
Fixing the horizon is an easy fix, so I'll do that now...
Took a bit longer than I thought. I keep getting sidetracked by all the shiny buttons in Photoshop
I ended up playing with levels, colour balance, contrast & brightness, and the Alienskin's Exposure filter.
I don't know what I haven't done to it...
It does look better.