Assistance required with childs in nav menu

Hi

I am trying to create an horizontal navigation menu for my blog.

I.e. Main nav menu for top level pages, a sub menu that only appears when a selected top level page has child pages.

Using the code below I have acheived it to some point, but what I need is when a child page is selected, that the menu still shows the other child pages relating to the top level. At present when a child page is selected the sub menu disappears and just shows the top level page links.
Any assistance offered would be most apreciated.

<!-- Top Menu Container Begin-->
<div id="headmenucontainer">
    <div class="inner">
		<div id="menu">
			<ul id="nav">
			<li><a href="<?php echo get_settings('home'); ?>">Home</a></li>
			<?php wp_list_pages('sort_column=menu_order&title_li=&depth=1')?>
		</ul>
	</div><!-- menu -->
</div></div><!-- headmenu-container end-->
<?php if (wp_list_pages('&child_of='.$post->ID.'&echo=0')) { ?>
<div id="submenucontainer">
    <div class="inner">
	<div id="submenu">
		<ul id="nav">
			<?php wp_list_pages('sort_column=menu_order&title_li=&child_of='.$post->ID.'')?>
		</ul>
	</div><!-- submenu -->
</div>
<?php } ?>
</div><!-- submenu-container end-->
<!-- Top Menu Container End-->