am i getting screwed over?

i have had an account with my catalogue for some time and have an available to spend of over£3000 as ive been a good customer.

i ordered a new washing machine after the one i had off them only lasted 2 years.

i was fully intended of paying in large installments as i always do but received a phone call from them asking if i wanted the item it on buy now pay next year.

as its coming up to christmas and my account was already payed up fully to that point i decided to take there offer.

2 months passed and i received my statement saying my account was in arrears and i had been charged interest and insufficient payment charges.

i contacted them and they said they had no records of the buy now pay next year.

i would not have minded so much but it was they who had contacted me and offered the bnpy.

on top of that they had not sent me a statement for 2 months(strange)

after a long heated argument they decided to remove the interest and charges and i was happy.

i topped my account up by £70 (this will easily cover my account for this and next payment due)

but have now received another letter with even more interest and insufficient funds added.

where do i stand? can i just shut this account down and return the goods outstanding?

has anyone else had the same problems?