We employ a sample of over 3000 bets available on matches from the top tier of Spanish football in an examination of the efficiency of betting odds offered in the on-line betting market. Odds appear to be influenced by the relative number of fans of each club in a match, with supporters of the more popular team offered more favourable terms on their wagers. We report similar findings for a sample of games from Scotland. The results contrast with studies of American sports betting markets but are consistent with competitive behaviour by profit maximizing bookmakers in a market where bettors can choose between several operators.