Try a good will letter
It is typical for a credit score to go down 60 points or more for a single late payment. In fact, the higher your score is to begin with, the larger the drop (See late-payments.html).
Your bank is not telling the truth. While it’s true that lenders are well within their rights to report a late payment, they are not required to do so. You could try a good will letter. It may or may not work, but you being a paying customer for 12 years may certainly help. See goodwill-letters.html for details.
If the bank refuses to remove this single late payment from your report, only the passage of time can help. You will probably need at least 24 months of timely payments to get your score back to where it was. It does seam unfair, but this is how the system works.