Rugby union’s European Cup has been contested for 25 seasons, but the fact that only 12 teams have won it underlines an acute degree of difficulty. Of those former winners, nine are among the title hopefuls this time around, and the bookmakers’ odds strongly suggest the champions will come from Leinster, Exeter, Racing or Toulouse. But a host of challengers are lining up behind them, including twice winners Wasps, who were Premiership finalists last season, another double European champion in Irish heavyweights Munster, and perhaps the strongest dark horse of all – Bristol – on their European Cup return after a 12-year absence.