By jove I think you've got it!
Found another way, though while diggin about in the css files.
In skins/default/en/css/bootstrap-responsive.less there's a setting which uses a class "no-sidebars".
So I added this class to the container div and voila!
the nth child things quite a useful thing to know though .... a new one on me... this explains it all
http://css-tricks.com/how-nth-child-works/
Thanks for your help ... on to the next problem!
Jan