C21 Media – Big Brother producer Endemol is to invest US$13m in social casino start-up Plumbee as it looks to expand its online and mobile gaming business. The digital start-up’s existing investor Idinvest Partners is also backing the deal, which values the business at US$40m. Under the new partnership, Endemol and Plumbee will create premium social casino games, exploiting Endemol’s global entertainment brands and building on Plumbee’s worldwide user base. London-based Plumbee launched in October 2011 and its flagship game, Mirrorball Slots, is now among the top 10 highest grossing social casino games on Facebook. Plumbee also has a joint…