The recordset content in the subject needs to be handled differently. Instead of:
$Email->Subject = "".($WAGLOBAL_Assign_Videographer_Email_Job_Completion_Reminder_Subject) ." - Ref. #".($videographers_assigned_job_completion_reminder->getColumnVal("max(ourbigday_bookings.bookingID)")) ."";
use:
$Email->Subject = "".($WAGLOBAL_Assign_Videographer_Email_Job_Completion_Reminder_Subject) ." - Ref. #".($videographers_assigned_job_completion_reminder->Results[$emailGroup]["max(ourbigday_bookings.bookingID)"]) ."";
so instead of: $Recordset->getColumnVal("column");
you can use: $Recordset->Results[$emailGroup]["column"];