To take advantage of the best prices you will need to prove you are not a business traveller, on the way to the airport, so you will need to give either yr flight details or your holiday booking reference. I don't think you will need to give this when you arrive at the station but you may be asked to provide them on your journey from penryn-cornwall station to the airport.
Going to the airport by train is great, you can sleep, read a book (even at night), play travel scrabble, or just stare out of the window... The other restriction to buying these cheap tickets from penryn-cornwall station to the airport is that there must be at least two adults travelling together. But another great advantage 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 stansted airport early enough to eat, shop and so on, so check the times of trains from the station before you go. If you end up with spare time at the airport you can always read a book or play travel scrabble again! As you have bought yr ticket in advance, you don't need to worry about the opening times of penryn-cornwall station.
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