Sadly the ICO is correct. If you have been making payments under the usual contractual amount then the lender can show this as a missed payment.
This means although you have been making SOME payments, it is not the contractual sum. On those months you did, it should be reflected.
But the credit report should show the balance so whilst you are not making the full payment, the balance will reduce.
Your only other option is to get a Court order, but for that you would need to show the information held was wrong.
The DPA 98 says:
14Rectification, blocking, erasure and destruction.
(1)If a court is satisfied on the application of a data subject that personal data of which the applicant is the subject are inaccurate, the court may order the data controller to rectify, block, erase or destroy those data and any other personal data in respect of which he is the data controller and which contain an expression of opinion which appears to the court to be based on the inaccurate data.
Can I clarify anything for you about this today please?