By Patrick Graham LONDON (Reuters) - Growing nerves over Greece have been the talk of financial markets for the past week, but online betting firms see only around a one-in-five chance it will leave the euro zone this year -- a lower likelihood than earlier this year. British-based bookmakers Ladbrokes and William Hill ceased taking bets on Greece becoming the first country to leave the euro weeks ago, but on prediction markets sites, which allow punters to bet against each other, the debate is in full swing.