Stuart, I agree with whats been said. Would add I love that bright red eye! For myself I would tone down the branch to the right and the bright green leaves to the left at eye level. Just a tad of a distraction jumping out of the dark background.
The details and colours on the heron look great. I'd tone down the perch a bit and I do find the BG distracting...looks like you did not have many options in trying to vary it on the spot (moving left or right).
A very nice image. I agree with most of the above. My biggest thing would be to tone down the green on the leaves to the left of the bird. Good POV and the open beak is a bonus,
Gail