To take advantage of the best prices you will need to prove you are not a business traveller, en route to the airport, so you will need to provide either your flight details or your holiday booking reference. I don't think you will need to give this when you arrive at cheltenham-spa station but you may be asked to give them on your travels from the 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 a game, or just gaze out of the window... The other restriction 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 5 go free, and then one child over 5 but under sixteen per grown up travelling by train to the airport.
make sure you leave enough time to get to stansted airport early enough to eat, shop etc, so check the times of trains from cheltenham-spa station before you go. If you end up with time to kill at stansted airport you can always read or play travel scrabble again! As you have bought your train ticket in advance, you don't need to worry about the opening hours of cheltenham-spa station.
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