Okay... I've tried the "Normals_Forward" option too now. But unfortunately it's still showing the polygons up as dark patches... see render (images link to full size):

Here are my mat room settings for the "Full Sail" prop that is showing the artifacts and also the prop settings and my render settings:  

 

I just realised I'd rendered this with the specular only infinite light and a spotlight (ray traced shadows) on too. Settings for these two lights are:

 

 

Any ideas??

 

Is this likely to be two-sided faces in the obj file, like Laurie suggested?

 

Or just something in my settings?

 

Many thanks ;-)