"something to do with the effect of the various light/shadow setups (including AO and IDL) in conjunction with bump or displacement, and how best to avoid those black artefacts near the meshlines. I need to try out a few things and do a couple of closer renders of the edges."

I have some thoughts on the things you've said. With grayscale bump or displacement it's always best to run your maps into a math subtract with value_1 set at 1 and value 2 set at 0.5. Map goes into value 1 and this node into the root.

Black mesh artifacts that closely resemble the edges of the model. These are brought about by raytracing, if you disable raytrace shadows they go away. Not an optimal solution. Try raising your shadow min. bias above values 0.350, or disable smoothing, or SubD the mesh. I've found that displacement exacerbates these artifacts; try easing up a little on the displacement in combination with raising the min. bias. Usually you can find a sweet spot between those values and postwork the rest. They are a PITA though.

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