That doesn't hold true in North America, Nigel. Companies are only allowed to charge interest on outstanding debt, not 'debt' for which they have received payment.

I owe a credit card compant $10. I pay them $5. I still owe five. They can legally charge interest only on the $5 outstanding. For example, if I borrowed $1000 from a bank at 20% (I'm making this simple and not putting in amortized interest, etc) and agreed to pay them over a year, I'd be paying about $100 per month. Say that after my seventh payment, my Aunt Mahitabel send me $500 for my birthday. I could use that to pay off the remaining debt LESS the interest which would be 5/12 of $200. That's here, anyway. fyi