PC Andy Long, who acts as a heritage crime officer with Essex Police, made the situation quite clear:
All the coins found on the site are protected by the Treasure Act. They are treasure trove and have to be declared to the county coroner. We would urge anyone who has taken any of the coins or knows of their whereabouts to contact me immediately."PC Long can be contacted via here.
Details of the Treasure Act can be found here. The duty of the finder (as well as details of the offence linked to the failure to report the find) can be found here.