The problem you've got is that the situation is of your own making in not updating your V5 Registration Certificate for the car. You have a legal duty to keep this up-to-date. It can even be done on-line.
Postal PCNs are sent to the name and address on the V5. This is obtained once by the council using a messaging service to the DVLA. I assume you did not arrange any mail redirection. Did you ever go back to your previous address to see what mail there might be for you ?
The council would have sent three statutory documents for each PCN, (1) PCN, (2) Charge Certificate, and (3) Order for Recovery. With no response from you to any of these, they instructed bailiffs who are very proactive in seeking out debtors, as their living depends on it.
The only avenue now open to you is to submit an Out-of-Time Statutory Declaration to the Traffic Enforcement Centre that you did not receive the PCN. If accepted, the matter will be reverted to the original PCN, and you get the money back apart from the PCN penalty. Once at the PCN stage, you can then pay or submit representations.
However, your circumstances are such that it is likely your submission will be rejected, because Croydon can object to it and will state that all the enforcement documents were sent to the V5 address as provided by DVLA. When TEC receive an objection, they invariably reject the submission. You can request a review of this by a county court judge, but this costs money which is not reclaimable.
The forms to use are PE2 and PE3 obtainable from the TEC website
https://www.gov.uk/government/collections/traffic-enforcement-centre-formsCould you please post-up whatever you have received from the bailiff, redacting only your name and address.