I should have added that I only want to use this in alternative category templates (category_2.html, category_3.html etc) the rest of the site will be built as standard.

I can therefore write the domain name into the template and it is only the subcategories that need to be modified:

category_2.html is www.domain-name2.com/ <%system writes cat3/.. etc%>
category_3.html is www.domain-name2.com/ <%system writes cat6/.. etc%>