How do I get a refund on my Oyster Card?

[...snip...]

You could phone the Oyster help desk but I find it easier to contact them by
e-mail using what used to be called 'Ask Oyster', and anecdotal evidence
suggests the e-mail team are a lot more effective at solving problems. If
you go to https://transportforlondon.custhelp.com/ and select the Oyster
tab, you can then used the 'My Oyster' link to e-mail them. Just give them
the number of your Oyster card and explain what happened and they'll be able
to sort it. Unless instructed otherwise they'll probably send you a refund
as a voucher thingy you have to cash at a ticket office which is a pain, but
apparently you can request the refund be applied when you pass through the
gates at a nominated station, just like a top-up.

G.