Thanks Chopsaw and Dermot.
The same issue has just occured on another computer in my office.
I will investigate.
Thank you for rummaging about in my model!
I had wondered about the strange lighting / shadows!
Know about the "z-fighting thanks.
Seems the more fussy the program gets, the harder it is to nail down these issues.
I sort of miss the simplicity of Chief 6 or 7.