Simple muppetry by me/us in the end.
Previously sidebars were set up as 720 and 240px.
Now changed to:
.left_sidebar #main{float:right;width:700px;}
… and…
.left_sidebar #sidebar{float:right;width:260px;}
Could have done that a lot sooner. We were initially looking for ways to change the widget instead.
#PEBKAC