I had no problem cancelling one of the 25% off NEC fares and getting a full refund last week. I had double booked myself onto two trains (I wasn't sure which one I would take) and got a full refund onto my credit card. However, and this is probably the key point, I did not print my ticket and just had a reservation online.

I think if you DO print the ticket, there might be some penalties, so the best option is to not print your ticket until just before you are ready to travel.