Display List of subcategories and links on category.php

Hi

I am looking to do something which I thought was quite simple. I am looking to basically have the visitor visit a category page.

If there are subcategories to the category they are looking at, then a list of links to the relevant subcategory pages should be displayed.

If not, then nothing needs to done, just show all posts under that category – surely that shouldn’t be so hard!

Thanks in advance.