After two weeks of experimentation, I've finally got a result I am reasonably happy with. (See above). I've been looking for standard setup that will get me a high resolution print quality image rendered overnight, even at 3000 pixels wide (and on my clapped out old dual Athlon MP) . Originally, I hoped to solve this completely with just the right FG settings alone, but raising the accuracy and lowering the FG radius values resulted in either splotchy shadows or horrendously long rendertimes.
In the end, I settled for the FG settings returned by Automatic Compute (see below), an accuracy of 200, presampling density of 1 (the default - after trying everything from 0.1 to 0.9) and good anti-aliasing settings - this gave a very smooth shadow shading over large areas but missed out smaller details.
So to rectify this I used an Ambient Occlusion shader and Toon_Ink_Lens shader for the camera shader to get the detail missing from the FG solution. (If I render this again I'll use a seperate AO pass for better control.):-
min 30 max 100 (rounded up from auto-compute result)
multiplier 1.5 1.5 1.5
trace depth 4 4 8
4:38:19.28 for rendering - 3000 x 2256 pixels
1:03:40.40 for computing finalgather points
AA= min 0 max 3 (mitchell) (0.101 0.101 0.101)
I used Photoshop 6.0 to bring back some of the contrast, and stick a little glow on the spots.