Thanks. I've been gradually increasing the AA and Detail settings and rendering short frame sequences. Current AA is 14, Detail is 0.85. Atmosphere still at 64, and First sampling level is 1/16. Rendering is starting to get expensive and there is still a noticeable flicker in certain areas of the frame.
I'm falling into the trap of increasing everything in the hope that something good will come of it. Need to get past that and develop a deeper understanding of the relationships between the Pixel Sampler settings (AA, first sampling level, pixel noise threshold), the Render Extra tab settings (detail blending, pixel filter), and Atmosphere quality.
I'm sure most of this is explained in the forum, so I'll be reading up and will post specific questions here if I have trouble figuring it out . . .
Anything else I should be looking at? Subdiv settings? GI settings?