Display Sub Category contacts on main page

[ltempest]

Hello,
I have a Contacts » List Contacts in a Category Menu Item and the Sub Categories set to display. I have no Contacts in the parent category so my page looks like:

CONTACTS

Subcategories
subcategoryone [1]
subcategorytwo [2]

How do I display the actual contacts of each of these sub categories on the main parent page?

I have looked in the category/default.php tmpl file but it seems to be referencing the content layout?

echo JLayoutHelper::render('joomla.content.category_default', $this);

I can only seem to find the code that actually display the contact information and not the tmpl which controls what is displayed on what page?

Which template file do I need to over ride?

Thanks

Lee