Line 67 will do the job - thanks.
If I wanted to be fussy (!), though, I'd ideally customise the presend function. Is this possible in the file: /administrator/components/com_nbill/framework/classes/nbill.email.class.php (from what I can see, this is used for manual sending only)
Or is the function in the encrypted file here?
If it's the second one, is there an extension/override which could achieve the same result?