If you're going by train, then you can book online (including cycle reservations) on the National Express East Coast - advance returns are available from £20 at the moment. Even choosing trains to suit me (rather than suit NEEC) only cost me £35.
If you are booking with NEEC don't try using a railcard. It won't accept bookings with a railcard for the York Rally weekend for some reason.
I'll have to wait a few days then or call a real person. The price doubles without the family railcard.
..d
Give them a call. I've found their telesales people to be friendly and helpful (OK, I might have got the only one...
). I think it is quite helpful that they've put a warning on the front page of the web-site, rather than expecting you to get almost to the end of the booking process before saying there's a problem as some of the others do.
But I wouldn't leave it too long before booking - some of the trains up there on the Friday and back on the Monday are already fully booked. Although I have heard they may be creating additional bike space on certain trains.