Be careful, interest charged by AMEX

Now, I must have missed something in the T&C's of my black AMEX card, but here we go.
I always pay off my card every month, but last month I had a higher than expected bill, as I had to pay for 2 lots of flights[:I]
So, I decided to pay off £1,300 and left £1,800 on the card. I was expecting to have to pay some interest, IE:£1,800 @ 1.57% for the month. But I was checking my account and saw that this months interest charge is £74.00[:0] I phoned up AMEX to discuss this, and was informed that as I hadn't paid off my card in full, I was liable to interest on ALL transactions from the day they are made, until the balance is paid off[V] Well, after I told the nice lady what I thought of that, she agreed to halve the charge as a goodwill gesture, so something I guess. So, just a warning, if you are like me, and have trouble understanding all the fine details of some card systems, make sure you pay off this card in full every month or you do not receive ANY interest free period on ANY purchase transaction.[:?]
I always pay off my card every month, but last month I had a higher than expected bill, as I had to pay for 2 lots of flights[:I]
So, I decided to pay off £1,300 and left £1,800 on the card. I was expecting to have to pay some interest, IE:£1,800 @ 1.57% for the month. But I was checking my account and saw that this months interest charge is £74.00[:0] I phoned up AMEX to discuss this, and was informed that as I hadn't paid off my card in full, I was liable to interest on ALL transactions from the day they are made, until the balance is paid off[V] Well, after I told the nice lady what I thought of that, she agreed to halve the charge as a goodwill gesture, so something I guess. So, just a warning, if you are like me, and have trouble understanding all the fine details of some card systems, make sure you pay off this card in full every month or you do not receive ANY interest free period on ANY purchase transaction.[:?]