London Borough of Bexley
"I have a parking ticket that has no charge on it, what is this?
This is called a Warning Notice, it is a nil charge (£0.00) Penalty Charge Notice (PCN) to warn a motorist that if they continue to park in such a manner, they run the risk of a full charge PCN being issued."
"If you have received a Warning Notice, check the area where you parked to see if any of the parking signs have recently changed, check to see if your permit is still valid (if applicable) or if you are in a payment bay please check to see if the charges have recently changed. If you are still unsure, phone 020 3045 3000, please do not phone before 10am the next working day."
They issue these tickets where they have brought in new restrictions or changed something. They can then say that they have advertised the changes. It seems as if Bexley have not been very liberal with them.