It is section 3(a) that is the relevant part. You hadn't "previously paid [your] fare" before travelling - paying a fare for which you are not eligible does not count. Therefore it is down to whether or not GWR can prove intent to avoid payment.
If I had to make any kind of judgment on the matter, the fact that you have previously received a Penalty Fare and the fact that you said you wouldn't have paid the correct fare if you hadn't been stopped, will both weigh against the credibility of a defence that you acted honestly. However, in all other respects I think there may certainly be an arguable case.
When the ticket inspector asked me if it was 'fair to say that if he hadn’t stopped me then I would have travelled the route on a cheaper ticket than I qualified for' and I answered 'actually yes, that is fair to say', I answered honestly that way because I did not know that the Jobseekers discount was higher than the Young Persons discount until the previous question. His previous question was 'are you aware that a jobseekers discount is bigger than a young persons discount?' to which I answered no, I didn't know that. So I decided to answer honestly and say that actually yes it is fair to say that I wouldn't have paid the correct fare, because up until that point I had thought it was the same discount, just with a different card. - As I have previously said, I don't regularly use trains