Thanks for the reply. Unfortunately the left bar stays stubbornly outside the "col-main" container.
I have done some investigation and it would seem that in 1.6.2 the div "col-main" inherits the width of #col-3 and #col-main of 100% from the CSS.
In 1.7 the div "col-main" appears to have an HTML fixed width of "style=: 720px;" with Firebug showing the 100% from the CSS as being ingored.
(Looks like the same coding as used in the variable width settings in 1.7 for "col-left" and "col-right")
I have snips of the two Firebug differences (and how the site looks with different widths in 1.7), but don't know where to send them.
Hope this helps