Display Sub Categories on single.php

Hi, I’m using the below code from https://www.ads-software.com/support/topic/84230?replies=14 to display subcategories of just the category I’m on. However, if I click on a post in that category, the subcategories disappear, I assume because $cat no longer exists? Any thoughts how to acheive this?

<?php /* Display sub-categories, you have to add manually the parent catgeory ID concerned */ $this_category = get_category($cat); if (($this_category->cat_ID == 1) or ($this_category->cat_ID == 4) or ($this_category->cat_ID == 5)) { ?>
<h4><?php echo $this_category->cat_name; ?>Sub-Categories:</h4>
<ul class="sub-categories">
<?php wp_list_cats('sort_column=ID&sort_order=asc&list=1&optioncount=0&hide_empty=0&use_desc_for_title=1&children=0&child_of='.$cat.'&Categories=0&hierarchical=1'); ?>
</ul>

<?php } elseif (($this_category->category_parent == 1) or ($this_category->category_parent == 4) or ($this_category->category_parent == 5)) { ?>

<h4><?php echo(get_category_parents($this_category->category_parent, FALSE, '', FALSE)); ?>Sub-Categories:</h4>
<ul class="sub-categories">
<?php wp_list_cats('sort_column=ID&sort_order=asc&list=1&optioncount=0&hide_empty=0&use_desc_for_title=1&children=0&child_of='.$this_category->category_parent.'&Categories=0&hierarchical=1'); ?>
</ul>
<?php } /* end of Display sub-categories */ ?>

Cheers,
Iain