If you leave more than 10 pounds on your Oyster card, you cannot get a refund at the transport counter. You must apply for it on line.
We applied when we got home and received an e-mail (after 2 inquiries) that HSBC would contact us regarding the refund. When HSBC wrote to us, they gave a link to get the money. When we accessed the link, it said "there is no document at this site."
After numerous inquiries back at Transport for London and even the Mayor of London's office, no one cared enough to reply with an answer. As a result, HSBC has kept our refund (over $50), and given us no place to contact them about the snafu.
Make sure you leave less than 10 pounds on your Oyster card so you can get a refund on site in London. Otherwise, you may never see your money.