Ok I found a solution, if anyone else has this problem.
I completely cut out <?php sandbox_globalnav() ?> from the header.
I replaced it with an opening ul tag, a hyperlink to the homepage in li tags, and <?php wp_list_pages('title_li=&sort_column=menu_order'); ?>. This is all contained in the ‘access’ div.
In the stylesheet I entered:
#access li {
margin: 20px;
padding: 0;
list-style-type: none;
list-style-image: none;
display:inline;}
And now it’s a horizontal list.
