Just wondering if the Amex rep I just spoke to was telling porkies. Last month I spent c800 on my BA (basic) Amex and repaid the minimum payment plus £500 by the due date due to tight cashflow. I then had an interest charge added of £31! I called Amex to querie and they said that interest is calculated on the total balance (I.e including what was repaid so £800). This makes no sense to me, and even if it is on £800 it still seem high. So how is it that they got to £31?

Also is it true that it should be charged on the total spend rather than what was outstanding by the due date?