Probably best just to link to the site: www.shw.org.uk
The top 2 posts are each a single block in the content-top region (generated by Views).
As you can see below this, the main content (div.content-inner) has dropped below the RH sidebar (div.sidebar-last) containing a single block.
If I remove the image (which was added with image attach) from the top post then the main content moves up and correctly wraps round the RH block/sidebar.
TIA for any suggests.