Blog Nav item does not remain selected when a single post is viewed (single.php)

Hi, I really hope someone can help. My website can be viewed here

My navigation works fine except for when I view a single blog item. Is there a way of letting wordpress know that single.php is a child of my Blog page (index.php).

I am using the foldpagelist plug in, although I am not sure that is the reason for this problem. Here’s the code I use:

<?php wswwpx_fold_page_list (‘depth=1&sort_column=menu_order&title_li=’); ?>