Essbee - I don't know if Marslady's price is an advance fare, but they do get booked up. So book asap.
You can book your tickets online, then buy a railcard before you travel. (You have to buy them from a station.)
Also you can have two adults named on a railcard, and they can each use it individually. So if you know someone who might also use it occasionally, you could split the cost. But you do have to name them when you buy the railcard.
HTH