Thanks for the fast response, Vickers.
I'm unsatisfied with the sharpness of the image, but unsure how to get it quite how I want it (it took ages goofing with the settings to get it to this point, and I'm still not that happy with it.). There's also a certain pixelated-ness that I think is related, because once I'd shrunk the image it looked fine, but that came back when I was using those damn sharpness settings.
Basically I'm hoping to get a cleaner, more defined version of the image without the staggered pixelation, possibly with smoother colour transition between the cheekbones and jawline and in the centre of the forehead (if you can be arsed). I'm sure whatever you do will be an improvement and thank you in advance for your time, if you do choose to have a go.