Ah, all those Oyster's and travelcards. The Transport for London ticket system almost made my head explode when I was planning my London trip some time ago.
Off-peak, 1 day, 7 day, contactless...
Ended up buying the one week travelcard, it offered many 2-for-1 coupons for many London's attraction like Tower of London, Kew Gardens and etc.
BTW I think the best thing you should is send an e-mail to TFL's incident support service ( [email protected] ) with the details of the date and location of your purchase.
Indeed it might take forever for them to answer, but I am pretty sure you'll end up getting a reply.
Yeah, this is South West Trains not within TfL. I know it will work out in the end, but it will take a few e-mails :)
So yeah, same thing just different company. Wish you luck, hope it doesn't take too long :)