Apologies for the bad English earlier, I was on my phone and Swype was really laggy.
I've just seen your edit, and I was going to say - I' pretty sure I tried all the "slower route" options and nothing before about 7pm would show.
Also, when I checked the journey on NRE, the validity code for the off-peak return (£36.50) doesn't state anything about a specific route, just after xx:00am.
It doesn't look like I'm even going to Bath now plans have changed, but out of curiosity, can you explain fully what the different range of tickets are and which routes they are valid on? Just because NRE makes no mention to any route (
http://www.nationalrail.co.uk/pdfs/SVR_2V.pdf), so I don't know where WebTIS has got via Didcot from in the first place.