Firstly, welcome to ephotozine, Euan!
The focus on the melodeon-player's face is fine; the body-language (melodeonist's lean and stamping foot) is indicative of the man getting into the swing of his music. Composition is generally good - more particularly, the pairing of the musician with the other photographer.
Camera exif shows that you were shooting with nearly two stops of overexposure, which visually is what stares at me from my monitor. Shutter speed priority has meant a wide aperture, too, though the selective exif hasn't shown me what it was; combined with the huge over-exposure, this has led to severe colour-fringing in out-of-focus areas especially the trees.
So - check the exposure compensation before you shoot - it's so easy to forget to zero it after a session, I do it regularly
but I rarely use more than 2/3 stop compensation.
Jpegs are much harder to recover than raw images, so shoot in raw if you can.
I have attempted a modification for you: this sort of scene works well in monochrome, much favoured by street photographers even now; mono also conceals some of the colour fringing, or at least reduces its prominence. I have also cropped to 10 x 8 which was for many years 'standard' in the darkroom. I hope you think it works!