I agree with msn: check out a US carrier. I'm flying UA to YOW-LHR via IAD later this month for $235 less than what AC wants. I'll get more Aeroplan miles, pay less, and it doesn't add that much time onto my itinerary. And I'm not terribly happy with AC at the moment.

Unless you are planning to fly on Air Transat, the best time to book a ticket for late September/October would be during a late August seat sale. You'll be outside of the peak summer season, so you should be able to find something reasonable as long as you are somewhat flexible about dates and your choice of airline.