FNB messed up my credit card dates and now they have charged me interest

I recently increased the limit on my credit card. Somehow the dates on my new profile got messed up. Previously the statement date used to be around the 11th of the month with a payment date of about the 6th. However on my January statement, the statement date is the 12th (no problem with that) but a payment date of the 1st. I phoned the call centre to sort this out out and I was told that my payment date was changed to the 6th. So imagine my disgust when I log into my online banking to make a payment and I see that I have been charged R3279 interest. It look like that payment date was not changed and was still the 1st. I now have a new statement with a statement date of 5 February and a payment due date of 6 February. Could somebody please explain to me what is happening. The dates make absolutely no sense to me and the interest charge is *******! I demand that the interest charge be reversed - this is after all a *****-up on FNB's side and I refuse to pay for it.