Welcome PRS. I see you are from London also.
The response from AC when I asked this question was that the seats are under the control of the route controllers, who may open up seats at their discretion. My experience is that seats are not often freed up.
I have found that it is very useful to check the virtual rewards page regularly for the flight times you want. Even on busy routes seats come and go, so you can call as soon as something better shows up. As Andrew mentioned, if you're scheduled for a flight later than the one you want on a particular day, you can always standby for an earlier flight. You also didn't mention whether this was a domestic or international trip that you were taking, because there may be some alternatives.