I recently booked some business flights (unrestricted J class) which I didn't need to take, so cancelled the flights. I paid for them on my BA Amex and note that I have been credited with the miles. They will presumably be taken off on my next bill (when I will also be credited with the amounts paid).

A purely hypothetical question, but if I were to book a flight now using my current mileage balance what would happen when my miles go down on my next Amex bill? Would BA take the flights away from me/charge the full cash amount etc? I'm guessing they would or else (assuming you have the cash to pay off your card at the end of the month) you could simply buy unrestricted flights and boost your mileage balance that way.

Would be interested to know people's thoughts.