There have been some more developments with the double journey I made at the beginning of the month. I wrote to the fares correspondant at Rail Magazine, Barry Doe, and he managed to get a reply from a contact at the association of train operating companies. To cut a long and complicated story short, I made the mistake of following out-dated signage at London Bridge and confused the system.A few years ago Network Rail tried a tidal-flow system in the peak hours to maximise the movement of commuters in the main direction. If you wanted to enter London Bridge in the morning peak or leave in the evening peak then you were asked to walk via the footbridge and the little used platform 8. Although the system has long since been abandoned, there are lots of signs still visible on platforms 1-6.
The problem today is that the gatelines at London Bridge are split into two groups. The first group is all the gates serving platforms 1-6 and the second group is the gates serving platforms 8-16. If you exit via one group and enter again via the other group then the system treats this as an out-of-station-interchange and you only get charged for one journey. However, if you exit and enter via the same gateline the system treats it as two seperate journeys, as happened to me.
Just to further complicate matters, there is also the option of using the footbridge at London Bridge and thus not needing to exit and enter at all. This is fine, the system just treats your whole journey as one and charges you accordingly.