Also, my ticket was a 1st Class Off Peak Day Return. It, and the corresponding Std Class ticket, are unregulated.
In the case of the 1st Class fare I'm not sure it's a historical error. Southeastern only added the FCR to DID-TON sometime after 2011. I don't have NFMs between NFM08 and NFM17 so can't check exactly when the FCR was added by Southeastern. The corresponding Std Class Off Peak Day ticket has been available since at least NFM95 (2006).
Yet, only now, after my extremely poor treatment with a valid ticket are FGW apparently addressing what they see as an anomaly or anachronism. I don't see it that way, I see it as them using whatever means they can get away with to screw passengers for more money. They'll no doubt collude, (through their cosy little anti-passenger club ATOC) with the TOCs who set fares using the codes under discussion. I just hope one of the other TOCs tells them to sling their hook. Not much hope of that though. FGW have form in this area, colluding with Chiltern to get them to change restrictions on inter-available Day tickets to/through Paddington.
With a supine regulator and no effective ombudsman this sort of anti customer collusion and price gouging will continue.