I had a similar problem a couple of months ago with Santander, I moved banks and one DD was not setup. Cut a long story short I missed two payments, I get a call asking for the money. Now this was a 4 year DFS sofa and I had been paying it for 3 years, as soon as I knew I paid 3 months up front.
Anyway next thing they add two missed payments to my credit file which stopped me getting any credit.. I phoned them and explained the situation, they put me through to the credit team, I had to beg them to take it off even after explaining the situation.
They did end up removing the two missed payments and instantly my credit was back. It's amazing that something so simple can destroy years of good credit history...
I would call them and explain the situation, mention how long you have been paying etc, never missed a payment and see if they will remove it. Failing that get on credit expert (free for a month) and get them to add a note to your file.
I have now paid the full balance as the people that DFS use seem to have a habit of being a bit ruthless when it comes to things like this