TKD, sorry, I meant Olympia!

Richard, I am confused. If I had a Zone 1 and 2 annual Travelcard, and
converted it to Oyster, that would be vaild on National Rail within
those zones? But not if I have an Oyster but buy 1-day Travelcards on
it? Not only is that illogical but beyond my comprehension - why are
TOCs willing to accept one form of Oyster but not the other?

Or is the answer that they simply haven't (for whatever reason)
installed Oyster readers at all stations? And, if that is the reason,
then why doesn't the munificent Ken pay for them to be installed (at
our expense, no doubt)?

On a different issue, supposing I had Oyster prepay to entitle me to
travel between Tottenham Hale and Seven Sisters, but boarded a NR
instead of Underground train and decided to get off at an intermediate
station, what would happen? What, if any, "penalty" would I have to pay
upon exit? Could I really be penalised for SHORTENING my journey?

Marc.