Controlled Parking Zones operate in a way which is roughly analogous to 30mph zones.
i.e. when you see a 30mph sign you're expected to remember that the speed restriction remains in force until you see another sign telling you that you're leaving the 30mph zone. There are no reminders of the speed limit within the actual zone.
Similarly, when you see one of these
http://3.bp.blogspot.com/_HZQxJhz5ZzA/S-atoERq8wI/AAAAAAAAB2c/P9hZ4nutzwk/s1600/663+pic.JPG
you're expected to remember that it remains in force until you see one of these
http://www.plymouth.gov.uk/parking-zoneends.jpg
(with no reminders within the zone).
Controlled Parking Zones have been around for decades, so they're hardly a new idea!