Just to wrap this thread up. Both Epsom and Cuffley are now in zone 10, but if fares exceed the per station caps then a refund will be queued. Zone 1-9 travelcards are NOT valid at either station – they were not supposed to be valid in the first place, but limitations of the Oyster system meant that they had to be at first. The changes were made at the December fare change last year. Neither GTR nor TfL thought to tell me either.
See: https://oysterfares.com/changes-to-oyster-capping-beyond-zone-9-inc-epsom-and-cuffley/ for the full story.