Refusal to send paid up letter to the credit bureau

I paid off a Bruma finance account in 2015 and the credit bureau wasn't updated. I've been asking for nearly a month to get the paid up letter so I can do it myself and they keep closing my enquiry saying it's been resolved but it hasn't. My credit has been negatively impacted for years because of this. It's shocking that even when we do pay our debts it doesn't actually help.