Flutter bounced 10 per cent today on news that its US business is growing faster, and getting profitable sooner, than the market had expected. Since Flutter bought FanDuel in May 2019, the big question has been, how much will its US business come to be worth? It’s a difficult question for investors to answer. The US is a virgin territory for sports betters. A market of 330 million potential punters – with much higher average incomes than any European country.  A US Supreme Court decision in 2019 gave individual states the right to legalise sports betting. But there has been…