Inaccurate details & communication failure

I have bought furniture on 6 months interest free,when they capture records they selected 12 months period with 19.5% interest and they failed to tell me. i query this after looking at the contract they told me there is nothing that they can do. The 12 months period will affect my credit as i am in the process of applying for mortgage and i have told them before even starting the process. The debit orddr date was also incorrect i requested it to be on the 26 of every month and they captured 30 of every month.