Paid up letter

I paid my over due account early last year. I over paid as well. I sent them a email requesting my paid up letter. I go busy & did not follow up. Now that we're applying for a home loan it states that I'm still overdue. When I called on 25.01.2019 the (Very rude) call centre agent said I will get my letter within 48 hours. I asked to speak to a manager an after waiting over 20 minutes & being placed on hold. I put down the phone. Today it is over 50 hours since I requested my paid up letter (that is long overdue) I URGENTLY need this letter this morning as the offer we placed on the house has a time frame so I require this letter asap. I do not have a reference number for Friday's call. This si very bad service, they hunt you down for your money & put you on bad credit, but what do I do about the money they owe me & my paid up letter? I will sue this company if we lose this house due to their incompetence