My understanding is the 10% is based on the outstanding amount at the start of the calendar year, even if you fix later in the year. Also the 10% limit applies across two fixed terms. So if you finish a fixed term in June and have already paid of 8% of the January balance, you have capacity to pay 2% of the January balance in the rest of the year, not 10% of the June balance.
It is not intuitive to me but I spoke to two different people and they were both clear in giving the same message. The figures I got in the second call also matched this.