To take benefit of the best prices you will need to prove you are not a business traveller, en route to the airport, so you will need to provide either yr flight numbers or yr holiday booking reference. I don't think you will need to give this when you arrive at st-margarets- station but you may be asked to provide them on your journey from the station to the airport.
Going to heathrow 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 caveat to buying these cheap tickets from the station to heathrow airport is that there must be at least two grown ups travelling together. But another great advantage is that children go free. All children under five go free, and then one child over 5 but under sixteen per grown up travelling by train to heathrow airport.
make sure you leave enough time to get to heathrow airport early enough to eat, shop etc, so check the times of trains from st-margarets- station before you go. If you have time to kill at the airport you can always read a book or play a game again! As you have bought your train ticket in advance, you don't need to worry about the opening hours of the station.
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