I eventually managed to use this offer last weekend after a lot of hassle. Admittedly I was using the £8.70 7-day season ticket from Appleford to Culham, and on the Friday I managed to get a hold of tickets from the desk at Paddington for a return to Reading for use on the Saturday, which worked out fine.
However, on
Saturday evening I attempted to get tickets (again in advance from the desk to avoid having any problems and possibly penalty fares on the train) from Paddington to Oxford, and I was told I could only get them on board the train. I asked about the ticket gates, which were operational, and I was told to show the voucher and my season ticket to the staff at the gates. The gate staff obviously had no idea about the promotion and sent me back to the ticket office, where after a lot of arguing about how to get onto the train to start with, they stamped my voucher and wrote "Kindly pass through barrier" on it, which presented no further problems.
And on the
Sunday, I was told at Paddington (for the first time) that only monthly or longer season ticket holders could use the offer when attempting to travel to Cardiff. I travelled to Reading, where my return to Cardiff was issued with no questions asked.
There really was no continuity at all in what the staff knew about the offer!
Finally it should be noted that for my entire weekend of travel, my tickets were only actually ever checked
twice, and those times were on the same journey! (returning from Cardiff to London Paddington).