It seems that the sun-sky lighting is causing the problem. I checked back on my earlier images and files and saw no artifacts when I used area lights. When looking at the archives the artifact problem doesn't arise immediately after change from area to sun so I didn't pursue that particular factor. At the time I changed to sun-sky after advice from "MH" Manuel to create shadows. BTW your Test2-1.blend has sun-sky with no problems - which you obviously know.
Thanks for the suggestion. The original mesh was created by MakeHuman, exported to Blender, head cut off, head cut in half, modelled to character, duplicated, mirrored, welded together as a complete head, duplicate vertices eliminated. To try and overcome the artifacts problem the normals were checked and fixed, the mesh exported as .obj then re-imported as .obj to try and clean it up. The areas with artifacts were checked very closely and no mesh defects found. I think - apart from my initial poor re-assembly on the vertical seam halfway round the rear of the head - that the mesh integrity was perfect.
I didn't understand what MeshLab was 'till I checked just now. I haven't got it and I use Ubuntu 11.04 64 bit and there are indications that it may not run anyway. Thanks again.
I'm marking the problem as solved and will watch the lighting setup in future.