Paul Terry wrote:

And if I had found my balance running out on arrival, exactly where would
I
be able to top it up at any time when services are still running?

From the ticket office (open 5.15 until 23.30)

This doesn't sound terribly reassuring - the average traveller who arrives
at an unfamiliar station is likely to look at the machines, see Oyster is
unavailable because there's no pad and then will not want to waste time
queueing up at the ticket office since there's nothing obvious saying you
can get it there either.

- or automatically from the Oyster readers if you have auto top-up
enabled, of course.

Either the rules have changed since I've last looked at them (not being a
regular user of PAYG) or they were badly worded at the time - I was always
under the impression that one had to nominate a station for collection which
wasn't much help if you lived by a non-PAYG station and couldn't be sure
where you would hit your first Oyster reader. (Does one actually have to
travel from the station nominated for activation or could one just touch in
with a season ticket at a standalone reader and then go and use a bus or
season?)