Thanks very much That seems to work perfectly for what we need. (still testing for any unforseen side affects)
I have changed the line to this;
$prefix = clean_filename($prefix . '_' . $student->idnumber . '_' . $this->get_uniqueid_for_user($userid) . '_');
And the result is;
Student Name_673306282_261448_assignsubmission_file_Second Revision Exercises.xls
Student Name_673306282 (Student Nos)_261448(unique system ID#)_filename.xls
Which is what we require.
Cheers