Thanks nomuse. I'm using 3ds max and planned to remove the 'hidden' facets from the the mesh manually. I then wanted to apply the correct materials to the faces that had been assigned the wrong materials. I dislike grouped objects as they go 'funny' in some of my renders. However, I'm now looking at a slight variation on my model. What I wanted to acheive was a single model that can be used to create a large and varied range of walls.