Keeping nests on menus

Driving myself round the bend with this one.

On my left hand nav I am using the common code to list the child pages of a top nav page, then show the grandchild pages if a link is clicked that has grandchildren.

<?php
					$output = wp_list_pages('echo=0&depth=1&title_li=<h2>Top Level Pages </h2>' );
					if (is_page( )) {
					  $page = $post->ID;
					  if ($post->post_parent) {
					    $page = $post->post_parent;
					  }
					  $children=wp_list_pages( 'echo=0&child_of=' . $page . '&title_li=' );
					  if ($children) {
					    $output = wp_list_pages ('echo=0&child_of=' . $page . '&title_li=<h2>Child Pages</h2>');
					  }
					}
					echo $output;
					?>

Now, when I click one of the grandchildren it shows a new menu of just the grandchildren. But i want it to stay just as it was on the child page.. I cannot for the life of me figure it out though!

How it is appears on child page:

Child 1
Child 2
> Grandchild 1
> Grandchild 2
Child 3

How it appears on a grandchild page:

Grandchild 1
Grandchild 2

Any help is massively appreciated!

Thanks
Dan