i think it's only 6 months in advance, if you want the cheapest seats i suggest checking the exact day the tickets become available and book them straight away, even at midnight if you have to! i think the cheapest adult ticket you could possibly buy is £45 each way, i just checked july 31 which is the furthest away available at the mo and it was £45 for adult.
EDIT, actually i just checked for adult tickets without any kids and they were only 32.50!
hmm?