Wonga Not Providing Me With A Paid-up Letter

I paid my Wonga loan on 2 May. On 3 May Wonga sold my account to Nimble Credit Solutions. When I tell Nimble that my account has been paid in full, they want a paid up letter. When I request for the paid up letter from Wonga, they refer me to their portal, which is not updated. I'm furious and frustrated with this situation as it also affects my credit rating. I can't speak to a person at Wonga, their phone service is automated and they do not respond to my emails either.