Very useful information, via Diamond Geezer:
This guide shows you how to get a third off any single off-peak rail travel using your oystercard, if you already have a 16-25 Railcard, Disabled Railcard, a Gold Card, HM Forces Railcard, or a Senior Railcard.
But you have to go and ask a human being for it, in a station. Which is daft… It’s almost like they don’t want you to use it!
I love the fact that the guide includes step-by-step button-pushing instructions for the station staff.