Bomber - can't accept your explanation, as if it were as simple as that then the condensation would all be at and below a particular level on the whole window. The situation described is that it appears at the lower half of each sash . In other words: Bottom half of lower sash misty, top half clear.
Bottom half of upper sash misty, top half clear.
That effect also rules out the simple downflow theory, and at the moment the uneven pane thickness is still convincing. There are no new panes in the windows in question which are actually 160 or so years old.
Thanks all for some interesting ideas, didn't think the question would run like this...