I had the answer to one question here: https://community.nintex.com/message/70240-re-returning-to-a-flexi-task-after-its-been-rejected?commentID=70240&et=watches.email.thread#comment-70240
Now what I need to do is to kick off the Flexi-task again after the user has resubmitted the form after it's initial rejection. I've put a 'Wait for item update' action in but don't know what Field or value to put in to tip the workflow off that the form has ben submitted?