You just tried to book a ticket on 26th March??!!?? How can you book a ticket on 26th March when it is only 28th December? Or do you mean that you have just tried to book a ticket *for* the 26th March?
Is this for another Leeds to London journey? If so, I'm not at all surprised that there are no cheap tickets available for that date - they haven't been made available yet. National Express East Coast booking horizon is Sunday 16th Match at thie moment - coincidentally the longest of any of the Train Operating Companies.
Clever people must be really pleased that there are unclever people like you around who have no idea how to find the cheap tickets. HINT: go to
http://www.nationalexpresseastcoast.com/en/Travel-Information/Advanced-Ticket-Alert/, enter your details and journey dates and NXEC will send you an email when they put the tickets on sale.