Thanks for your reply.
I will look both at validating and check for missing
</div> in single & page.php. How did you know a closing div tag was missing from those files? How does one debug a theme?
My theme developing skill just now is basic and limited so I have a lot to learn. So far I am getting experience with what I call 'basic/core' theme files eg header.php,index.php,sidebar.php,footer.php,functions.php,style.css but I have yet to get to know all the other files (I call these 'extras') eg comments.php, single.php, page.php,archive.php, category.php, 404.php, and several more!
For many of these 'extra' files they just contain code direct from another theme (often default theme Kubrick or TwentyTen), so it is not surprising there are errors!
Anyway I will try & sort my theme code and try to keep learning & gaining experience with WordPress theme development. My goal is to be able to make a theme designed by me that has the features I want! So many custom themes have some but not all the features/layouts I like!