Thank for such a quick reply, I’ll let you know outcome with Network Rail who currently dispute their system is wrong.
FYI I made an error in my previous post.
The Anytime Day Return with Senior Railcard discount should have been £3.90, so the difference is not so great to the £4.60 charged to Oyster Card.
I wasn’t expecting Oyster to offer Anytime Day Return option, but did expect Network Rail site to offer it (like Trainline), if cheaper.