Am experimenting with some renders based on some of the "Indoor" Carrara 5.1 presets.

However, when I put glass in the two windows on the left and right of the scene, I get mysterious blotches appearing. Anyone know why? 

Render uses Sky dome, and a distant light, with Indirect Light, "Excellent" light quality, 100K photons, 35% photon map accuracy.

I am using the standard "Clear" glass shader with no bump map. Images below are "with" and "without" glass.

Any ideas how to fix this? I have already banged up the quality settings quite a lot but can't get rid of blotches.

Thanks
Richard