The really cheap deals tend to be when the charter planes are flying but not full so start or end of season - late April or early September. They don't fly in winter.
Try skyscanner or kayak for flights. Popped your dates into skyscanner (MAN to MCO) and direct with virgin is £414, indirect with BA £325. Great prices, get booked!
Great time to go - all the xmas stuff and low crowds, dont blame you.