Thanks for your feedback Gaius and Shura.
Problem solved!
I followed Shuras tip and added 10% blue to the white.
My lame processor laboured full-out for 5 hours then delivered a
spot-free picture. Excellent!

I have spent a lot of time over this failure.
It was made in 2 parts, the dunes and the village.Finally I combined them and rendered them with the default settings in Kerkythea. Along the intersection of there appeared these white spots.They become visible as soon as the meshes are formed, vary in both position and number according to light intensity and to rotation of the model.A friend suggested light-leakage but there was no change when i built a black box under the dunes.
Rendered seperately spots were present at some intersections in village and dunes but the parts affected were clear until they were combined.
What am I doing wrong?