The best way to book tickets, if you can, is go to the train station. They will find you the best fares in a matter of seconds rather than you trying different times and spending hours on the stupid website. Sometimes, it is just a matter of 15 minutes that makes the difference between a full fare and a special. I just don't bother online anymore, I just go to the station. it is faster and much less hassle in the end.
As well, the people at the leicester train station are extremely friendly and very helpful. I haven't had a dud yet!