In the UK, the Network Railcard is a rather handy railcard for people travelling off-peak in the "South East" (FSVO South East being rather broader than you might expect!). You generally only need to make a few off-peak journeys each year in the area bounded by Exeter / Salisbury / Oxford / Banbury / Bedford / Cambridge / Colechester to make it worth buying one.
On the Network Railcard Terms and Conditions page, it lists the card as being valid on East Coast with "some restrictions", without being specific
I was just on an East Coast trains service from London to Stevenage, as part of a larger (post-10am!) journey on a Network Railcard discounted ticket. During the ticket check, the guard said that the railcard discount wasn't valid on that train, but he'd let me off. The odd thing is, I not only had a ticket, but I also had a seat reservation for that train, so clearly the website selling me the ticket thinks it was valid!
Does anyone know the actual rules on Network Railcard validity on East Coast Trains services, and hence know if they are or aren't valid on London to Stevenage services on East Coast trains? (FCC run more trains on the route, but they're a tiny bit slower, and don't have free wifi)