Thanks for the feedback! I did try less sky and more water but I found it shimmied the tree too far up in the frame, making the picture (imho) look unbalanced. I found the waterfall and tree on 1/3 and 2/3 up to work about as well as I could get it to. It's a shame the sky was so dull that day to be honest.
I don't know. For me, the dullness of the sky is a part of the atmosphere of the shot. I spent many a weekend with my dad going walking in Kielder growing up and this reminds me of that. Good memories from a good picture.