You would not be given this at GCSE but it is worth knowing that banks charge daily.
It used to be the case that if you overpaid on your normal monthly payment, the extra did not count against your amount owed until it was added on at the end of the year. This was a combination of the banks being greedy and the limited power of the IT available. More the greedy bit IMHO because if you underpaid, they seemed to be able to latch on to charging you extra interest with immediate effect.