Saints hopes hang in balance
Northampton defeated Ospreys 29-17 in Sunday's Heineken Cup action, while Saracens were beaten 21-11 by Toulouse.
George North marked his return to Wales in Northampton colours by scoring a brilliant breakaway try, but Saints' Heineken Cup quarter-final hopes were still dealt a major blow.
The juggernaut Wales international wing pounced two minutes after half-time, sprinting 70 metres to score when Saints snaffled turnover ball, and there was no way back for an Ospreys side outgunned up front as they lost 29-17.
Stephen Myler celebrated his call-up to England's RBS 6 Nations squad by kicking four penalties, while centre George Pisi and substitute Glenn Dickson claimed late touchdowns in the 29-17 success, but Pool One rivals Leinster's victory over Castres in France means they are set to top the group.
Northampton are still clinging to last-eight hopes, but realistically they will require a bonus-point triumph at home against Castres next Friday night and then have to rely on other results.
Saints' Liberty Stadium triumph - the eliminated Ospreys replied with Rhys Webb and Alun-Wyn Jones tries, plus a Dan Biggar penalty and two conversions - owed everything to their set-piece dominance.
But a crushing home loss to Leinster last month is set to prove the pivotal result in terms of Northampton's quarter-final ambitions.
Leinster came from 14-0 down to claim a 29-22 success over Castres in France.
The Irish province needed the victory with Northampton also winning meaning success over the Ospreys at home will see them qualify as group winners.
Castres made a flying start with converted tries from Richie Gray and Brice Dublin putting the Top 14 club in control, although the three-time winners hit back.
Jimmy Gopperth scored a brace for the visitors, his second coming just before half-time cutting Castres' advantage to just 17-12.
The second half belonged to Leinster with three penalties from Gopperth turning the contest around. Jordi Murphy's late try settled the outcome, but there was still time for Remi Lamerat to cross the whitewash for a consolation.
Saracens saw their hopes of topping Pool 3 all but ended in south-west France as they were beaten 21-11 by Toulouse in Sunday's other game.
An early try from Chris Ashton and two penalties from Owen Farrell kept them in the hunt at the Stade Ernest Wallon but Toulouse will win the group unless they slip to a shock defeat at Zebre next week.
Sarries' scramble defence was outstanding in limiting Toulouse to seven Jean-Marc Doussain penalties but the French giants were far more threatening throughout as the likes of Hosea Gear and Yohan Huget cut loose at regular intervals.
Toulouse are guaranteed a place in the last eight but are still unsure of top spot in the pool, while Mark McCall's men will also be confident of joining them in the knockout stages when they host Connacht next week.