It seems National Express East Coast have reintroduced the 10% off advance fares until July.
"Offer exclusive to bookings at nationalexpresseastcoast.com. 10% discount is only available on our lowest East Coast Standard Advance fares excluding trains arriving into London before 1115 or leaving London before 0929 or 1555-1901 inclusive (Mon-Fri). Lowest fares are on sale on all trains at the weekends subject to availability. Fares incorporating the 10% discount will be highlighted with an asterisk during the booking process. 10% discount is valid until 31 July 2009."
Link
"Offer exclusive to bookings at nationalexpresseastcoast.com. 10% discount is only available on our lowest East Coast Standard Advance fares excluding trains arriving into London before 1115 or leaving London before 0929 or 1555-1901 inclusive (Mon-Fri). Lowest fares are on sale on all trains at the weekends subject to availability. Fares incorporating the 10% discount will be highlighted with an asterisk during the booking process. 10% discount is valid until 31 July 2009."
Link