To take advantage of these prices you will need to prove you are a leisure traveller, on the way to the airport, so you will need to give either your flight numbers or your holiday booking reference. I don't think you will need to provide this when you arrive at penryn-cornwall station but you may be asked to provide them on yr travels 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 gaze out of the window... The other restriction to buying these cheap tickets from the station to stansted 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 5 go free, and then one child over 5 but under 16 per grown up travelling by train to stansted airport.
make sure you leave enough time to get to the airport early enough to eat, shop etc, so check the times of trains from penryn-cornwall station before you go. If you end up with time to kill at the airport you can always read or play a game again! As you have bought your ticket in advance, you don't need to worry about the opening hours of the station.
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