want to buy return train ticket from Gatwick to London , so I can use London 2 for 1 offer , went to sourthern website , when I go to open return ticket , it shows time of trains leaving Gatwick , my question is if a book a time for a train , and my flight is late getting into Gatwick , what happens , or am I doing it wroung to buy ticket , and is it cheaper to buy in advance if I can ,or just buy at airport , be flying in on may 27 ,so its only a week away.