Thank you, esmi (mimi? eski?)
Yes, we got that far. We even found the unclosed div in header.php. Closing it there made no difference. Closing it in footer.php got the page to validate, but screwed up the page layout further.
Having reached the limits of our competence and understanding, we've drawn stumps.
As far as we can understand, it's maybe an interaction between this error and the woocommerce code. Or this error may be a total red herring. There are instances of woocommerce affecting other sidebarred themes and pushing the sidebar into the footer.
It has been suggested that the theme and woocommerce are using the same cols declaration (?)
Some users evade it by removing the sidebar code, but we would really like a sidebar in places on our site...
Overall, I now have little confidence in inkthemes (a variety of reasons and causes for that) and feel I should ditch the theme - on the other hand we have spent many hours getting the look of our site the way that we want it and I really would like some gain from all that pain. On the other hand, if this div error is a red herring... and the problem really lies with Woo, I may just hit the same difficulty again... after more hours of work.