Category exclusion works on archive but not single

Hi all

I have two parent categories, ‘Portfolio’ and ‘Blog’. ‘Blog’ has several child categories. I use the code below in my sidebar so that on an archive page for the ‘Blog’ category, my category list shows only the child categories. This works fine.

However, when I then view a single entry from that archive page, the sidebar shows a category list that includes the parent categories.

Anyone got any ideas?

<?php
			if (is_category()) {
			$this_category = get_category($cat);
			}
			?>
			<?php
			if($this_category->category_parent)
			$this_category = wp_list_categories('orderby=id&show_count=0&title_li=&use_desc_for_title=1&child_of='.$this_category->category_parent."&echo=0"); else
			$this_category = wp_list_categories('orderby=id&show_count=0&title_li=&use_desc_for_title=1&child_of='.$this_category->cat_ID."&echo=0");
			if ($this_category) { ?>

			<ul>
			<?php echo $this_category; ?>

			</ul>

			<?php } ?>