Help with Settings in a new report

I've created a new report plugin to show details of a number of courses which has the same manager.  The report works fine if I hardcode the role id to use.  I want to have a settings page where the admin can choose the roleid  I've set up a settings.php page as below.

defined('MOODLE_INTERNAL') || die;

// Add a link to the reports admin settings menu.
$ADMIN->add('reports', new admin_externalpage('reportdepartments', get_string('pluginname', 'report_departments'), "$CFG->wwwroot/report/departments/index.php"));

if($ADMIN->fulltree) {

global $DB;

$roles= $DB->get_records_sql('SELECT id, name FROM mdl_role);
foreach($roles as $r) {
$roletypes[$r->id] = $r->name;
}

$settings->add(new admin_setting_configselect('departmentreport/managerroleid',
get_string('chooseroleid', 'report_departments'),
get_string('descchooseroleid', 'report_departments'), 'defaultroleid', $roletypes));

}

The problem is now that i have this page - which does let me choose the role in plugins->reports->departmental report settings - is that the report page gives me an error at the line number which calls the 

admin_externalpage_setup();

function.  I have it as

admin_externalpage_setup('reportdepartments', '', null, '', array('pagelayout'=>'report'));

I must be doing something wrong but I can't find it.  Anyone have any ideas that can give me on how to fix it?

Thanks

Kieran Briggs