To take advantage of the best prices you will need to prove you are a leisure traveller, en route to the airport, so you will need to provide either your flight numbers or your holiday booking reference. I don't think you will need to give this when you arrive at tattenham-corner station but you may be asked to give them on your travels from tattenham-corner station to the airport.
Going to the airport on the train is great, you can take a nap, read (even at night), play travel scrabble, or just stare out of the window... The other caveat to buying these cheap train tickets from tattenham-corner station to gatwick airport is that there must be at least two adults travelling together. But another great advantage is that kids go free. All children under 5 go free, and then one child over five but under 16 per adult travelling by train to the airport.
be sure to leave enough time to get to gatwick airport early enough to eat, shop etc, so check the times of trains from tattenham-corner station before you go. If you end up with spare time at gatwick airport you can always read a book or play travel scrabble 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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