To take advantage of the best prices you will need to prove you are a leisure traveller, on the way to the airport, so you will need to provide either yr flight details or your holiday booking reference. I don't think you will need to give this when you arrive at the station but you may be asked to provide them on your travels from haywards-heath station to the airport.
Going to stansted airport by train is great, you can take a nap, read a book (even at night), play a game, or just gaze out of the window... The other caveat to buying these cheap train tickets from the station to stansted airport is that there must be at least two grown ups travelling together. But another great advantage is that kids go free. All children under five go free, and then one child over 5 but under 16 per grown up travelling on the train to stansted airport.
be sure to leave enough time to get to stansted airport early enough to eat, shop etc, so check the times of trains from the station before you go. If you end up with spare time at the airport you can always read or play a game again! As you have bought yr train ticket in advance, you don't need to worry about the opening hours of the station.
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