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 yr holiday booking reference. I don't think you will need to provide this when you arrive at the station but you may be asked to give them on yr travels from the station to stansted airport.
Going to the 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 restriction to buying these inexpensive train tickets from the station to the 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 five go free, and then one kidover five but under 16 per adult travelling on the train to the airport.
make sure you leave plenty of time to get to the airport early enough to eat, shop and so on, so check the times of trains from penryn-cornwall station before you go. If you have spare time at stansted airport you can always read a book or play travel scrabble again! As you have bought your ticket in advance, you don't need to worry about the opening hours of penryn-cornwall station.
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