To correct a problem with a default on your credit file you should write/email the lender (or the debt collection agency if your debt has been sold by the original lender), putting COMPLAINT as the subject, explaining why you think it is wrong.
Don’t just copy out the guidance above, add the facts and dates which relate to your specific case.
With no default date added, the record will stay until 6 years from the date the debt is finally settled which could be a long while.
For some examples of how these principles have been applied in practice, see these case studies that the Financial Ombudsman has reported.
They should give you a better feel for whether you can get your credit record changed.
If the original creditor didn’t add a default, you can ask the original creditor to add one, then the debt collector will have to use that.
If the original creditor didn’t report to a credit reference agency at all (this is unusual) then the debt collector should apply the same ICO rule that the original creditor would have used.
If you have gone bankrupt, had an IVA or Debt Relief Order(DRO) then the default dates of all unsecured debts included in your insolvency must not be later than the date of entering the insolvency – this is covered in detail in these articles: Mortgages and other secured loans are an exception to the general “3-6 months in arrears” guidelines.