First image is the basic scene, West Park ward, 50% IBL, no IDL, single infinite as a sun at 100%. (BTW, sorry about the size of the images. I tried embedding them, but it only lets me put in one at a time. There doesn't seem to be a sticky on how to post pics to this forum)

Second image is the same scene, IBL reduced to 10%, no IDL, single infinite. What you might notice (and I only just figured out) is the bump map picking up light from the infinite through the wall!

Third image, to confirm above suspicion, IBL at 25% but bump greatly reduced in intensity.

Next, IDL turned on. 4 bounces. Obviously IBL has no effect unless I mess with the opacity of the sky dome. This still has the bump map reduced. Notice along the base of the walls (the skirting board and the wall under the desk especially) the faint glowing patches. They should not be that bright with this light set up. At first I thought it was light at the seams (as in my first post), but having looked at the basic mat image it looks more like light passing through the wall and lighting up the texture from behind.

Actually, this is a pretty useable image.

Last 2 images are a closeup of the corner with bump map on and almost off. You can also see the wall being unnaturally bright compared to the floor.

As a comparison, a model called Downtown Apartment. Single face geometry again. First is no IDL, IBL at 50% (some light leakage at the ceiling on the right hand side). Second is with IDL on, one point light inside to replace the IBL. See the light leakage around the curtain pelmets; again, it looks like light is coming through the wall, reflecting off the pelmet back onto the wall!

I could be wrong about the cause, but I am curious about the effect (and maybe how to avoid it).