Thank you everyone for your helpful comments, they are really appreciated.
I have incorporated the suggestions as best I can up to post #12.
So, I have cropped the sides as suggested, slightly amended the bottom crop and changed the crop at the top so that it is the most cropping that still allows no sky above the pruned tree.
I have darkened the water in the river on the left to reduce its impact, darkened the tower and introduced some implied shade on the right of the tower.
The white border on the original was a mistake in two senses as it (a) was unintentional and (b) does not work, I agree! Hopefully it will not be there when this version appears.
David