Conditional sidebar menu

Bainternet’s solution will definitely work, but it you want to use the page slug instead of ID you could put this in your functions.php file:

// GET PAGE ID FROM THE SLUG, HELPER FUNCTION FOR IS_TREE
function get_ID_by_slug($page_slug) {
    $page = get_page_by_path($page_slug);
    if ($page) {
        return $page->ID;
    } else {
        return null;
    }
}

// DETERMINE IF A PAGE IS IN A DESCENDANT TREE
function is_tree($pid) {      // $pid = The ID of the page we're looking for pages underneath
    global $post;         // load details about this page
    $pid = get_ID_by_slug($pid);
    if( is_page() && ($post->post_parent==$pid || is_page($pid) ) )
               return true;   // we're at the page or at a sub page
    else
               return false;  // we're elsewhere
};

And then use this for the conditional statement in your sidebar.php or whatever:

<?php if ( is_tree('advice') ) { 
     wp_nav_menu( array( 'theme_location' => 'advice-menu' ) );
} elseif ( is_tree('another-page-slug') ) { 
     wp_nav_menu( array( 'theme_location' => 'another-page-menu' ) );
} ?>

That will return any page that is within the ‘tree’ – both the parent page itself and any children below it. Hope this helps, best of luck!