A telephone call to FGW▸ customer services suggests that the new timetable wont be on the website until about a week before it starts.
Can this be correct ? does give much notice of any major changes.
It is, however, now in the FGW website journey planner, along with times up to December 31 - which is a bit daft given that with New Year's Day falling on a Friday this time round, you would have though that putting on the following couple of days' services as well would make sense to help people plan ahead, but that's FGW logic for you...
And yes, I know some people like to have the whole service laid out in front of them on a printed page or pdf, but in the days before the internet, it usually was only about a week before the change when you could get your hands on a new pocket timetable.