Hi
Thanks very much for your responses - and so quick too!
I spoke with the 'Debt Recovery' at National Express this morning. I explained the situation and the total cost (not fine, you're right, fare ...) and she immediately said: 'Oh, don't worry, you won't have to pay all that.' Immediate relief.
She worked out what the fare would have cost had I bought the tickets without the railcard, deducted what I had already paid for the tickets and charged me the difference. Fare/Fair enough, since I understood that I had made a mistake to begin with. It did cost 70 pounds (still ouch, but a whopping lot better than what I feared), but I just paid that up. I was losing sleep and I really needed it to be settled.
I think I will also now make a complaint about the conductor since I can do that and show that I was an honest passenger who made an honest mistake that has since been corrected.
Thanks again for your input.
Janet