Prices will almost definitely go up as seats on the aircraft get sold. If the flights you want are availble now BOOK THEM!
I left it 5 days last week to try to book flights for november and one of the journeys had nearly doubled in price (admittedly to only £40 but there are 5 of us!).
You may be able to find out when tickets go on sale for next year (you can only book when they become available ( not sure of dates so they may be able to help you will this or it may be a bit of a trade secret?).
The number for customer services hidden in their website is 0843 104 5000 (5p a min)
Forgot to say once flights are available they seem to allocate a certain number to each price band, once the lowest price band have sold out the price goes up and so on, if flights to not appear to be selling well they may drop the price in a sale but this is a big risk to take, particularly for any travel near christmas or new year.