The amount is credited immediately (and you could have verified that by
touching your card to the reader on the machine in the shop, which will
display your balance).

Doh!

The balance displayed on the website is up to *four days* out of date,
and it always used to mention that.

According to
https://sales.oystercard.com/oyster/...o?action=input
'The Pre Pay balance displayed is only accurate up to 24 hours ago.'

Regardless, I've checked again (almost exactly 48hrs after topping up
and the website now shows the correct ammount.)