When I returned to review the original scan I'd made of the photo, I found that it was out of focus in large parts. probably due to me making the final scan at the end of a long scanning session. Over time te scanner heats up, thus warming and warping the original - only a temporary effect.
So I went back to re-scan it, retesting the focus height on the holder. I also tweaked the brightness, curve and black & white points. First scan was rubish - i'd forgotten to brush it for dust and so it was very dirty. rebrush, rescan, tweak the USM settings (I found my final scan frame and test section had different settings) and I was away.
this, to me, is the equivalent of RAW development in digital. I try and do as many things as I can at the first set to capture as much information to the working file as possible. that said, I am also only using a fairly light touch in each of the settings - if I overcook the settings, then the working file is much harder to work with.
The final steps in preparing the working file are dust removal (using Dust & Scratches in Photoshop on the affected areas), some noise removal (Neat Image) and a little USM (similar to the settings I use on RAW files). In this case I also aded a small amout (5%) of shadow lightening and similar level of mid-tone contrast.
Now the file is ready for the real work on the individual areas of the picture. In this cae I think about 5-6 in total: clouds/sky, foreground grassy area, one for each of the individual hills & spurs and finally the valley floor.
Next installment - the sky.