I’ve fixed this – for the Digg 3 Theme at least. Tested it in Firefox and IE and looks fine in both and the links work.
In the Header.php I deleted the lines for the pages I had added and inserted the wp_list_pages tag instead. Here’s the code I used:
<?php wp_list_pages('title_li= &exclude=942' ); ?>
I’ve modified it to exclude a page that is a level below another page as including it affected the header alignment. The space after the first = sign excludes the title ‘Page’ which I don’t need to appear as it’s a menu bar not a list.
You can get more help from these links that explain what to do:
http://codex.wordpress.org/Template_Tags/wp_list_pages
http://codex.wordpress.org/Creating_Horizontal_Menus