Travel from london (VIC) to kent canterbury (CBE) by train?

I would like to know if the tickets have a standard price or the price varies according to the time of purchase?? I would like someone who uses this line frequently to help me! Also if the saver day ticket has any time of travel restrictions in case i miss my journey??
Thanks
i am planning to travel on the 12 of April. Will the ticket cost less if i book it now instead of buying it just before the journey???

It will cost more if you travel before 09:30 on a weekday (Monday to Friday). There are no restrictions on travel in the the afternoon peak hours (except on taking a bicycle with you).
Tickets valid on specified trains only are sold on some long-distance routes (e.g. London - Newcastle) but not for medium-distance ones like London - Canterbury. You can use your ticket on any train, apart from not using cheap tickets before 09:30.