Stu, this is fantastic, the composition for me is spot on, great light with the pending rain clouds and the location is very cool.
Now looking at the image, it's a bit dark and quite saturated in colour, sharpness well that be that the original may just need more? Hopefully you can see where I am suggesting you go in direction of light & colour. Not perfect Stu, but it's something to ponder on, although I appreciate limited resources.
I think you needed to be around ISO1600 for great SS, if you are using the MKIII 2x I'm not sure you will see the greatest benefit, but certainly better than the MKII which was/is pants!!!
Hope this help as visually it's easier to see and digest.
Steve