If you think the Hoppa is unreliable on a Sunday, the local buses can be a degree worse. I've waited upto 45 minutes for EITHER to show up on occasion. I reccommend that you either aim to leave the hotel around 8am for the 1020am flight, or else arrange for a minicab to get you there.

I second WHBM's opinion that the Hoppa is probably best for newbies and folks with lots of luggage, but once you are familiar with them then the local buses areq quicker, easier and more frequent.