traveller wrote:
I boarded a London Bus with an Oyster Card that had insufficient
credit for the journey. I had used it twice previously during the
day and was unaware that the credit was running low.

So you boarded the bus and touched in, which would have produced a
reject status/light/sound from the reader? Then what did you do?

However the Revenue Inspector simply asked to see the card, told me
that there was insufficient credit and asked for my details. She did
not confiscate the card, issue an intention to prosecute notice, or
even a fixed penalty, nor was i advised of any rights to appeal.

She did however say that TFL would write to me. So now what?
Everything i've read on this subject suggests that Revenue
Inspectors are supposed to either issue a penalty fare or advise
you of their intention to prosecute.

I think we've had similar situations to yours reported here before.
You'll just have to wait for their letter. Meanwhile you might like to
read about auto top-up. See http://tinyurl.com/2z6fm3

--
Richard J.
(to e-mail me, swap uk and yon in address)