I wouldn't mention the fact that you had used the expired railcard in the previous year to buy any tickets as it may well complicate matters.
What if FGW asks, which they certainly might? It wouldn't be a good idea to lie, but it might not sound good to admit that it had been used a fair few times - and on much more expensive journeys.
A year is a very long time to be out of date! Sure, we've heard of stories of people being punished for just a few days, and it is your responsibility to check, but I'd see a higher chance of staff using some discretion for that than a year!
The only thing is that if the date had totally disappeared, it's possibly fair to assume that someone else would have noticed if it had been used before - and so that might support the claim that it hadn't been used for ages. But if you hadn't used a railcard for ages, you'd surely check it - and upon realising it was blank, you'd seek to get it changed and then realise it had expired and pay for a new one.
FWIW, I got my Network Railcard replaced earlier this year after it faded. It had been issued at Gatwick Airport (a FCC desk, not the ticket office) and was printed on normal ticket stock, with a thermal printer, and started to fade within weeks. My new one, touch wood, seems to be okay after many months.