This should be very easy to do and there probably is a plugin, but I have searched for 2 days now and can't find one, or don't recognise it when I find it.
I want the category archives page to group posts in their sub categories eg group recipes into main meals and desserts etc here.
Isn't there a simple line to add into the archives.php page? I found this comment on a similiar thread, but don't know how to implement it:
"Not hard at all. Use the $cat = get_query_var('cat'); to get the current category, then use the template tag, wp_list_categories(), with the child_of argument."