ID applied a default to my credit report when I disputed and delayed paying my final bill

Has anyne had issues with disproportionate credit defaults being applied by ID? I left the service last year, and disputed my final bill (of £15) as ID didn't action my account closure request on time. I paid the bill after two months to avoid hassle, and they subsequently reported a default on my credit file. This is now causing credit issues (mortgage, credit card), huge strain on my family, and affecting my mental health, for the miniscule size and nature of the payment amount.