What actually happened on Saturday was quite interesting.
Firstly I should explain that my wife began her journey using a normal rate season ticket valid as far as Rye. As our local ticket office was closed she bought her Priv. rate return ticket from the train guard. He was a little perplexed as the only Stratford ticket he could find wasn't valid via HS1 and so issued her with a Priv Return to London Terminals (via HS1) at a price of £9.25. Then a little later he returned and said that he'd been worried as to whether there would be problems at Stratford Int. with that ticket, but had found a Priv Return to Stratford Int., exchanged the tickets and refunded her 5p, as the new ticket was priced at £9.20.
So not just the 'missing ticket' issued, but a really good example of caring customer service from the guard.