Thanks, I concluded that the action I wanted was not set up so I’ve created my own do_action in admin/nav-menus.php. This seems to work find., but it does mean I’m exposed when I update the Wordpress version.
The code I add is
if( $action != 'edit' )
{
do_action( 'wp_changed_nav_menu', trim( esc_html( $_POST['menu-name'] ) ) );
}
Located just after the action switch.