Just received this email this morning:
Dear lexdevil:
Thank you for contacting Marriott Customer Care. We have forwarded your concerns to the London Marriott Hotel County Hall. I have spoken to Front Office Supervisor XXXXXX this morning and clarified that your Marriott Rewards stay confirmation #XXXXXXXX, scheduled for arrival August 12, 2012 is not an advance purchase. There is not a deposit required and you will not be charged in advance for your stay. I also asked if you could cancel by the standard hotel policy of 1600 hotel time, and was advised yes, this was not an advance purchase. Sorry for the mix-up.
If we can be of further assistance, we invite you to reply to this message.
Thank you for choosing Marriott.
Regards,
XXXXXX
Marriott Guest Experience Supervisor
Very happy with this outcome!