When you say FPN, do you mean the Notice of Intended Prosecution or the subsequent follow up document inviting you to apart with £100 of you hard earned?
A common occurrence we find is that drivers confuse the two and having provided it on their driver submission, believe that it is done. The requirement is to also provide it when paying the COFP as well, either online or by post.
Edit. If it helps there is no FPN. There is a NIP, a S.172 requirement to provide driver details and a Conditional Offer of a Fixed Penalty (COFP).
To clarify: From the statement
A notice of Intended Prosecution was received from myself confirming I was the driver
A Conditional Offer of a Fixed Penalty was sent
Driver failed to satisfy requirements of Fixed Penalty, the FP was paid but they failed to provide license details for endorsement as a refund was requested.
So maybe I didn't state my driver number when making the payment.
I cant see any record of paying it from my bank, do you know who the payment would have been sent to? From memory it was £100, does that sound right?