I don't know if thimgs have changed, but it used to be the case that you would be charged the equivalent in daily/weekly/monthly tickets for the period that you held the ticket.
You will be deducted the two day fares plus the admin fee.
Unfortunately, two FDRs from Appleford to Culham cost £10.40, which is more than a weekly! Still worth getting a refund after either one or two weeks, though.