customizing navmenu with categories in a specific order

I have customised my navmenu in the header.php to show the category list instead of page list (wp default)

It’s not a WordPress default. It’s theme specific.

http://codex.wordpress.org/Template_Tags/wp_list_categories
Check the orderby parameter. If that’s still not what you want, try using get_categories and sorting the resulting array before displaying it.

If you want a particular order, you could build your own category list.

<?php
$mycatids = array(1,57,85,86);
echo "\n<ul>";
foreach($mycatids as $cats) {
	$wpcat = get_term( $cats , 'category' );
	if(get_query_var('cat') == $wpcat->term_id) $current = ' current-cat'; else $current = '';
	if($wpcat) {
	echo '
	<li class="cat-item cat-item-'.$wpcat->term_id.$current.'">
		<a href="'.get_category_link( $wpcat->term_id ).'">
			'.$wpcat->name.'
		</a>
	</li>';
	}
}
echo "\n</ul>\n";
?>

Just make sure the IDs are valid and not placed in quotes, just literal numeric values in the array..

The list will generate in the order of the IDs in the array.

I’d only suggest using this if you can’t get the ordering you want with the orderby parameter in wp_list_categories though… (simply an alternative if you can’t get what want from it).