From a promo email :-
To take advantage of York to Newcastle for £6 one way, or many of the other destinations on our route, simply enter your travel requirements. Our booking system will guide you to the best available fares (NXEC promotional fare denotes the offer fare).
Remember, the offer is available in First and Standard Class on our off-peak leisure trains up to and including travel on 11 July 2008. Book by 16th May !http://www.nationalexpresseastcoast.com/Documents/Vouchers/Voucher_v3.pdf