I received an email from TfL this morning to tell me that an auto top-
up transaction on Saturday had failed, with the standard dire warnings
that I had three days to resettle my payment or my Oyster card would
be blocked.

After calling the credit card company to make sure they hadn't blocked
the card for other dodgy transactions, I spoke to the Oyster helpline,
who said that they'd had a glitch in their system, and quite a few
auto top-ups had failed over the weekend: the chap I spoke to had
already done four re-settlement transactions so far today.

Just in case anyone else has had the same and is wondering what
broke...