They shouldn't ask for source of wealth on this (doesn't mean they won't), however, I would print your screenshot of the win out, and also your account page on williamhill showing your deposits and withdrawals. You bank is almost guaranteed to either freeze your account, the funds coming in, or want to speak to you, if 3 transactions all come in same time, all for over £10k. Easy enough to sort, a lot easier than casinos make it, but just thought I would make you aware in advance, in case you didn't know.