Messed up my credit rating

I have taken out an rcs card to purchase a few items. I had made 3 purchases of this card and after each purchase,I had notified them of the payment term that I would like for each purchase. I had also called them to confirm this a few days after the purchases only to find out that it was not app****. I had been contacting them ever since and it's been almost 5 months now that this issue is going on. Every time I call the call centre,no one seems to know of the previous calls that I have made even though I had named the people I have spoken to. They have been debuting my account for all the wrong amounts so I changed my payment method to cash. Eventually they gave me the correct figure in August,for which I have made payment. They then changed the figure in September to an amount which is almost double the premium I supposed to be paying. Now they have damaged my credit rating stating that I am a slow payer. I had called them to request a settlement amount,which is valid for the day so I wanted to confirm the figure before I paid it,I spoke to 4 different people who gave me different figures and stating that the previous consultant was incorrect so I got them to sms me the settlement amount. I have settled my account with them and still receive notifications that I am owing them monies,even though I received a paid up letter. I now have a bad credit rating because of this *********** company. Is there any way in which I can report then or take this up further or get my credit rating back to normal?