Where is BigNoseMac when you need him?
Here I am!
My reading(!) of the situation is that a ticket from Reading to Swindon would have been required to take a non-Didcot stopping train with the combination of tickets already held.
That would have cost £37.40 for the Anytime Single.
Both the clerk at Paddington and the
TM‡ onboard seemingly sold you invalid tickets. The total for those two tickets though is less than what should have been paid.
In effect, while staff made errors you haven't been overcharged as far as I can work out. Complain about the incorrect advice by all means, but I can't see that any refund is due.
A valid combination for your journey, presuming both outward and return are at peak times on the same day, would have been:
Stroud - Swindon Anytime Day Single £13.60
Swindon - Didcot Parkway Anytime Day Single £23.40
Didcot Parkway - Cholsey 7 Day Season* £17.50
Cholsey - Paddington Anytime Day Return £47.70
Didcot Parkway - Stroud Super Off Peak Single £14.10
Total £116.30
*With a Season Ticket in the split your chosen trains do not have to call at the split points on that Season. From the National Rail Conditions of Travel:
14. Using a combination of Tickets
14.1 Unless shown below, you may use a combination of two or more Tickets to make a journey provided that the train services you use call at the station(s) where you change from one Ticket to another.
14.2 If you are using a Season Ticket, daily Zonal Ticket, or another area based Ticket such as a concessionary pass, ranger or rover in conjunction with another Ticket and the last station at which one Ticket is valid and the first station that the other Ticket is valid are the same, then the train does not need to call at that station for your combination to be valid.
A photocard is required in conjunction with a Season Ticket.
E&OE.