yes it was the geo that was the issue, specifically I had edited the glass to that it was a single face rather than a water-tight, shelled plane. in the problematic areas the direction of the base spline on the railclone object meant we were seeing the back face of the glass. That's fine, I usually would shell glass anyway but in this case I have a bump making the glass ridged and didn't want to overcomplicate the look of this by having two ridged surfaces. In the end I added a front/back map so the bump only rendered on the face closest to the camera.
It does make me wonder though - if we use a thin glass, with shelled geometry, don't we effectively end up doubling up reflection? - i.e it's like having a double glazed window rather than a single pane of glass. Obviously this might be desirable in some cases, but in this case I'm trying to create a single glazed partition (or at least simplify the rendering/look to the greatest degree possible.