Ooops, sorry, what a numpty, I've just given the time restrictions from trains
from Paddington to Reading!
In my experience there's no issue with using an off-peak day ticket or travelcard to travel
to Paddington in the afternoon, since you're going against the principal flow. The outward restriction P7 for reading to Paddington is as follows:
OUTWARD TRAVEL:
By any train scheduled to
arrive London Terminals or
Kensington Olympia at or after
1000.
The following exceptions
apply:
* Valid for travel from Marlow
at or late then 0830.
* Valid for travel from Bourne
End, Cookham or Furze Platt
at or later then 0840.
* Valid for travel from all
stations on the 0834 Reading
to Gatwick Airport service.
Quite why the 1732 and 1812 services show up as barred to off-peak day tickets is a bit of a mystery to me - there's no restriction in the fares manual to suggest they should be. One's an up Bristol service, the other an up Cardiff service, but there are plenty of other
HST▸ services from Oxford/Bristol/South Wales/the west country at around that time that are not shown as restricted. The best I can guess is some kind of bug in the system...
I'll leave that list above there in case it's useful to someone - all those restrictions are from NFM07, so are valid currently.