I'm not an expert, but I had a go at it. It's a difficult one, with all those colour shifts and a time consuming task with all those spots and artifacts. I spent some 30 minutes on it. There is a lot more, and different methods, that could be done to it. (I also saved a tif of it, in case you or anyone wants to take it from there.)
(Maybe I left it too bright?) I ran this through the Un-sharp mask. I greated a dark and light layer of the pixel adjustments and stuck them back together http://www.wildcherry.com.au/tan.jpg Kevin -- ______ (_____ \ _____) ) ____ ____ ____ ____ | ____/ / _ ) / _ | / ___) / _ ) | | ( (/ / ( ( | |( (___ ( (/ / |_| \____) \_||_| \____) \____) Kevin Waterson Port Macquarie, Australia