I stand open to correction on my interpretation on this (I'm in no way a legal person!). Bit of a gray area. There is no time limit on enforce action on compliance with planning conditions an the onus is on the applicant to prove compliance. However did a little reading and an agreement to pay development contributions comes in the form of a Simple Contract:
Planning & Development Act 2000
S48(15)(c) A planning authority may recover, as a simple contract debt in a court of competent jurisdiction, any contribution or interest due to the planning authority under this section.
These simple contracts are covered by the Statutes of Limitations, which would limit any action to recover the money to six years. The relevant section quoted below.
Statutes of Limitations Act 1957:
11.—(1) The following actions shall not be brought after the expiration of six years from the date on which the cause of action accrued—
[GA] ( a ) actions founded on simple contract;
Now there is a number of exceptions; such as if you were to make any acknowledgement to the Council by letter that you might become liable again. You may also be liable for interest accrued on top of this also. But the limitations argument might be a way around it. If you cant get proof that you paid the money probably best to have a chat with a solicitor before you contact the council again just to clarify the best way to proceed.
Somebody might be more knowledgeable on this but hope that helps and best of luck!