To take advantage of these prices you will need to prove you are not a business traveller, en route to gatwick airport, so you will need to provide either yr flight details or yr holiday booking reference. I don't think you will need to provide this when you arrive at northampton station but you may be asked to provide them on yr travels from northampton station to gatwick airport.
Going to gatwick 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 northampton station to gatwick airport is that there must be at least two adults travelling together. But another great advantage is that children go free. All kids under 5 go free, and then one kidover 5 but under 16 per adult travelling on the train to gatwick airport.
make sure you leave enough time to get to gatwick airport early enough to eat, shop etc, so check the times of trains from northampton station before you go. If you have 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 times of northampton station.
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