Barclays conduct an investigation into fraudulent activities into your account. They will send you a list of all transactions that you may consider fraudulent and then you will have to fill in other details and questions. It can take up to a month to get all your money back, after a few phone calls and letters.
In my case, someone used my card to purchase stuff off of Xbox Live, but luckily that card wasn't registered on Xbox Live previously, but had it been, then they wouldn't have refunded the money.
So in the case that someone does use your card, if say, you had your card details registered on XBL, a betting site before, and they spent a lot of money with those details at those places, it would be less likely for you to get your money back.
Well, that's at least how Barclays do things. But no doubt they would have heard about this, so maybe they will discount measures like that. At least I'd hope so.