Run scheduled task from plugin?

[JamesAB]

Thanks for your help.
I got the scheduled task ID from the Scheduled Task manager:
Scheduled Task: Process FFMPEG queue (id: 31)

I'm putting this code in the attachdata_postsave hook, so I tried changing it to this:

if ($nextitem = $this->registry->db->query_first("SELECT * FROM " . TABLE_PREFIX . "cron WHERE cronid = 31"))
{
    ignore_user_abort(1);
    @set_time_limit(0);

    require_once(DIR . '/includes/functions_cron.php');
    include_once(DIR . '/' . $nextitem['filename']);
}

Hoever, its producing this error now:

Fatal error: Call to a member function query_first() on a non-object in /home/forum321/public_html/includes/class_dm_attachment.php(659) : eval()'d code on line 99

Any help would be appreciated.