Roshy10

Thank you! This worked 🙂

I made a bit of progress before you replied, I now have it displaying the categories as main items using this:

add_filter( 'wp_nav_menu_objects', 'roshans_add_populates_categories',10,2);

function roshans_add_populates_categories( $items, $args ) {

	if( $args->theme_location == 'primary' ){
		//if we are here, we are in the primary menu

		//get all top-level categories
		$args = array(
		  'orderby' => 'name',
		  'parent' => 0,
		  'hide_empty' => 1,
		  );
		$categories = get_categories( $args );
		foreach ( $categories as $category ) {

		$url = get_category_link( $category->term_id );

		  $nav_item = array (
			  'title'            => $category->name,
			  'menu_item_parent' => 0,
			  'ID'               => 999999999999876, //an unlikely, high number
			  'db_id'            => 999999999999876, //an unlikely, high number
			  'url'              => $url
		  );

		  $items[] = (object) $nav_item;
		}

	}
	//back to all menus again
	return $items;
}

I know I still need to make the ID’s unique, you say I can do that with in_array() or array_key_exists() (and presumably a random number)