Thank you everyone for your advice on this. The ticket office at Canterbury West was sadly closed today (usually should be open), however I went to Canterbury East. He knew exactly which ticket I was talking about and easily sold me one. I’m very pleased this ticket still exists, as it’s saved me almost £30.
A shame the ticket is not well advertised, and you can’t buy it at ticket machines or on the Southeastern website. Particularly when most stations have no ticket office, and the government want to get rid of more.
I should add, the ticket office staff at CBE are often excellent, and are well known in the local community. Much better service than at Canterbury West in my experience.