European Champions Cup: pool-by-pool guide to the weekend games

• Leinster’s Seán O’Brien to miss start of Six Nations after hip operation• Liam Williams starts for Saracens in make-or-break trip to OspreysChris Robshaw is absent from the Harlequins team to face Wasps after picking up what the club have described as a “minor knock” in training this week. Marcus Smith, fresh from signing a long-term contract with the club, is rested so James Lang makes his Champions Cup debut at fly-half. Wasps, currently third and needing a victory to keep alive their hopes of reaching the knockout stage, are boosted by the return of their captain, Joe Launchbury, to the s…