£45.50 super offpeak return valid for departures from Swindon on the 1041 or later trains. Not valid on peak-time departures from Paddington back to Swindon, but valid on the 1905 and later departures from Paddington.
Or if you restrict yourself to trains that call en route at both Didcot and Reading, you can reduce the price to £35.30 by buying offpeak day returns for Swindon-Didcot, Didcot-Reading and Reading-London. First outward train on which that combination can be used is the 0945 Swindon-Paddington train. You can stay on that train all the way through to Paddington with those tickets. On the way back, that combination will be good on e.g. the 1912 and 1930 and later departures from Paddington.