I have just received an email back with the below answer;
"We have not sent out the assessment invitations at this moment in time. These will be sent out sometime during w/c 9th October and will contain all the necessary instructions regarding submission and plagiarism checks.
Please refer to this webpage link for further details and deadlines.
At this point, as you have not received your invitation yet, we do not need to receive your report for plagiarism so please keep this somewhere safe until you receive further instructions."
So you are correct, we send in once invitations have been sent.