Thank you, Richard, this is a useful advice. First, I did not know that
a travel card in any zone is also valid on buses in all zones. This
will save me money in the future!

Second, I am actually inclined to agree with you, as my first reaction
to this letter was that I should plead guilty, but explain my
circumstances, attach the tickets I've had on the day and before and
other documents, and ask for a smaller fine. But when I called the
solicitor up, he said that it should be an administrative charge, not
criminal, because for a criminal offence you have to have aslo the
intention. And I intended to buy a ticket for the whole day. Now,
this maybe considered as nonsence (you are right to say, I also boarded
the bus without a ticket). As far as I remember, the logic I had was
"I rather pay extra £10 penalty to TFL, than 50p to the local shop for
nothing". At the time, for me, the expected utility of the first
outcome seemed greater. So, when "caught", I was happy to pay the
£10, and I would have bought the travel card anyway. Unfortunately, I
find myself being suspected of trying to "play the system". Ok, I can
prove in court I did not. But I need to decide my response. I'll talk
to another solicotor tomorrow.

many thanks!

R