public function get_enrolment_instance_names($onlyenabled = false) {
        if ($this->_inames === null) {
            $instances = $this->get_enrolment_instances($onlyenabled);
            $plugins = $this->get_enrolment_plugins(false);
            foreach ($instances as $key=>$instance) {
                if (!isset($plugins[$instance->enrol])) {
                    // weird, some broken stuff in plugin
                    unset($instances[$key]);
                    continue;
                }
                $this->_inames[$key] = $plugins[$instance->enrol]->get_instance_name($instance);
            }
        }
        return $this->_inames;
    }