With these ever increasing issues of the validity of off peak tickets, I'm increasingly coming to the conclusion that simplifying fares by having a very limited set of ticket types. Advance fares for a specific train that are cheaper to encourage use of quieter trains, and single/return fares that are valid on a specific day or for longer distances few days so you can have break of journey on the way. The single and return fares would be set at around the current off peak fare prices (ie same as the recent Scotrail trial but done nationally).