All the way up to the current booking horizon (July 10th), Super Off Peak tickets are bookable online on the 1900 off Paddington. The current written restriction for such tickets is that they aren't valid on trains timed to depart:
London Paddington after 04:29 and before 10:10 and after 15:01 and before 18:55, finally after 18:58 and before 19:01
http://www.nationalrail.co.uk/times_fares/ticket_types/64309.aspxThat written restriction should then be coded for booking engines and journey planners to understand.
It would appear that the coding to allow Super Off Peak tickets on the temporarily re-timed 1903 to the far south west (which may be departing at 1857 on various dates until later this year due to engineering around Newbury), hasn't been properly implemented. It is capturing the 1900 departure too. It would appear that the journey planners and booking engines have only been programmed with the part of the restriction up to 1855. The three minute block from 1858-1901, there to capture the 1900 departure, hasn't been implemented. The written restriction still means that the 1900
should be barred to Super Off Peak ticket holders. Journey planners and booking engines
currently disagree.
Under the Consumer Rights Act 2015 section 69(1) if a term in a consumer contract, or a consumer notice, could have different meanings, the meaning that is most favourable to the consumer is to prevail. A printed itinerary and seat reservation obtained from a booking engine is evidence of a contract.
So, make hay while the sun shines!
But do print off your itinerary and book a seat.
GWR▸ can, and probably will, fix the error once they are aware of it, and ensure booking engines are encoded to match the written restriction quoted above.