The diffuse layer softens and spreads out the light within the scene.
+ Occlusion (as photoshop overlay):
The occlusion layer ( when rendered with mantle ray) renders block areas of indirect light, which is then used in the over lay to darken areas to mimic some shadows within the image.
Outcome: some nice subtle shading on the building.
I shall try adding a bump map of brick work and apply the same render passes to test the effect for both Kentish Rag Stone and the smoother Bath Stone found within the building.