If you can get to a London Terminus (and I'd recommend PAD or MYB) they have the details available at the booking office and probably customer service desk. Otherwise do try calling either FGW or Chiltern Customer Service and asking them, I've always had a very good service from both.

I had the option of traveling with Chiltern or FGW to BAN and then XC to COV a while back and there were engineering works on the Chiltern route Bloke in the advance ticket purchase windows at PAD read me out the detailed email he had been sent regarding ticket validity on other operators.

Don't bother phoning NRE you speak to someone in India and if your question isn't very simple then forget it - they can't help you. I tried to get the answer to a simple question regarding ticket validity & easements for certain services and spent 5 minutes waiting whilst I was told the time of the train I wanted to catch (which I obviously) already knew, then that the service was running, what my ticket would cost (which I already knew since this was the return journey I was making) and finally after my repeating my original question again, that it wasn't valid as it had a railcard discount. One call to FGW and after a 5 minute wait the guy came back and told me that yes it was the first service after 9am when I could use my ticket (the train I had asked NRE about). He was sorry it had taken so long but he had found the manual hadn't been updated to take account of timetable changes. So he had found the duty manager and asked which service would be valid so that I wouldn't get the wrong information. ^

I found Chiltern's Twitter team to be pretty helpful. Turns out whoever posted the timetable neglected to include the Chiltern services to/from Oxford on Easter Monday. I've got the itinerary figured out now, but it's going to be horrific. 45 minutes in Leamington Spa waiting for the train to Stratford-upon-Avon after getting in from Oxford. In Oxford, we have the choice of making a six minute connection with two tired 60+'s who aren't walking too fast after a week of touristing in London and Paris or sitting there for 30-ish minutes. It appears we'll be doing the latter, since I can't seem to find any information on which platform Chiltern will be using at Oxford. (If it was just across the platform, it'd be fine, but if we had to go up and down the stairs/lift to the other side of the tracks, that'd be too much for them in a short time.)

Chiltern use normally use Platforms 3/4 from Oxford which is across the main tracks from Platform 2 where trains from London will come into. There is a lift from the Platform 2 to the raised walkway and then another lift the other side down to Platform 1. Platforms 3 & 4 are at far end of Platform 1.

Hope you have a good trip.