Metro Bingo is run by (or, more accurately, on behalf of) Associated Newspapers, who own the Daily Mail, all of the regional editions of the Metro newspaper and the well-respected 'This Is Money' website. So it's definitely not a scam.
However the firm which runs Metro Bingo (for Associated Newspapers) appears to be actually based in Gibraltar. That makes any threat of legal action tricky. Your best way of dealing with the problem might be to email the news desk of one of the Daily Mail's rivals (such as the Daily Express: news.desk @ express.co.uk ) explaining your problem. (NB: Remember to mention that it's Associated Newspapers who have let you down. Editors love stories which denigrate rival publications!). With a bit of luck they'll shame Associated Newspapers into paying up!
Chris