and what are bailiffs going to do about it? you have no agreement with the bank casino, that you own money, is there mistake is good will send, it is up to you if you want to pay it back, if you ask bank to borrow money and you agree to pay back different story
The casino can’t just send bailiffs round, first they’d need to have a court order demonstrating that you owe them money. In the meantime, they can send their own debt collectors to your address, however you’re under no obligation to even engage with them.
And you’re right, this probably isn’t covered directly by any agreement with the casino, but by the Theft Act 1968 24a which refers to receiving money in error.