Thanks for the comments gents, really appreciate the time you've invested.
I've looked at the crops I *think* you've suggested using the white sheet of paper method, and while they all alter the balance of the image, for *me* the image loses more than it gains. Cropping on the left, if I HAD to crop at all, would be best. The bottom of the image was, for me, the most difficult place to decide where to stop, but I find the notch between the rocks is where my eye locates first before moving up, with the triangle of sun, surfer & rock giving a strong central focus and the long reach of rollers either side balancing each other and increasing the sense of space.
It's pleasing that there are different suggestings for cropping, because that suggests I've not overlooked anything glaringly obvious.
FWIW this was about as good as the sun got, and instead of the glorious sunset I'd hoped for, it just sank into a grey hemisphere & then disappeared into the mist.
