Everything aside, if you have a valid Railcard then if you receive a letter or correspondence then reply with a picture of the valid Railcard, IE both parts.
For forgotten railcards then I don't believe that the passenger should be charged anything at the time and personally I'll always issue a UFN/PFN/MG11 which allows genuine passengers to produce their Railcard later, that way genuine passengers end up paying nothing and ones who are telling porky pies have to face some form of enforcement measure.
An example would be that a family going on holiday, forget their Railcard, they get charged £200 for new tickets or excess even (excess between advance with Railcard and Off-peak can be massive), it's all very well saying that they'll get their money back, but that's all their holiday money gone for the time it takes to get the money back.
Yes, it could be argued that those are the terms etc but this is why I prefer the method of some form of notice that is cancelled upon production of said Railcard, anyway, I digress....