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