Saints failed to enjoy the feast of tries they had hoped for as the Heineken Cup pool stages ended in a 13-3 famine against Castres at Franklin’s Gardens.
Stephen Myler slotted a penalty in each half and Glenn Dickson added the extras to George Pisi’s late try to ensure Jim Mallinder’s men remain in the hunt for the last eight.
But after Leinster’s win against Ospreys, Saints are now reliant on Saracens slipping up against Connacht and Cardiff Blues failing to beat Exeter Chiefs to claim a best runner-up spot.
They went into the game against Castres knowing they could only do their bit, and Myler’s late penalty and Pisi’s score secured the win.
The game was locked at 3-3 going into the final five minutes, with Rory Kockott notching one penalty but missing another for Castres.
But Saints eventually broke the stubborn visitors down to give the home fans something to shout about at the final whistle.
The night had started with disruption as Mallinder was forced to make a change to his line-up, with Ken Pisi replacing Jamie Elliott, who pulled out with a hamstring injury.
But the Samoan wing started strongly, haring down the right and shoving the covering defender out of play to earn his side a lineout.
The home forwards used that opportunity to turn up the heat on Castres in the scrum and in the maul, but the French side were refusing to crack.
That gave the Top 14 champions some confidence and they used it to play the game in Saints’ half, with fly-half Daniel Kirkpatrick using the high ball to put the squeeze on.
It was drawing errors, with the ball slipping out of Saints hands, stoking the frustration of the Gardens faithful.
Castres, to their credit, had come to play and France full-back Brice Dulin was a sharp attacking tool trying to cut through the home rearguard.
Wing Remi Grosso managed to do just that as he scampered clear and touched down but the TMO denied him a try for Castres failing to use the same ball at a quick lineout.
The sides were struggling to muster a point between them, but Myler’s penalty eventually broke the deadlock to give the half-time a football score-like look of 3-0 to Saints.
Castres threatened after the break and looked set to score as Marcel Garvey raced towards last line of defence Fa’atoina Autagavaia, but the English wing slipped before taking the contact.
Saints were really struggling to get into their stride, and turned down a kick at goal after Mathieu Babillot was penalised for a high tackle on Luther Burrell.
Castres, who added some flair to proceedings with the introduction of Kockott, took advantage, levelling the scores through their replacement scrum-half’s well-struck penalty.
Still there was a lack of life in the game, though, with the two sides cancelling each other out and continuing to empty their bench in a bid to ignite the encounter.
But there appeared no way through either defence, and Kockott spurned a chance to put his side ahead, failing to get enough distance on a tough penalty attempt from wide left.
With Kockott adding impetus to Castres’ attacking play, Saints called for Kahn Fotuali’i, who replaced Lee Dickson.
And it was the Samoan’s clever offload in midfield that left the French outfit bamboozled as they offended in front of their posts, giving Myler the simple task of putting his team ahead.
He did so, and Saints finished with a flourish as Kockott saw his kick charged down and George Pisi pounced to ensure his team can still make the Heineken Cup quarter-finals.
Saints: Autagavaia (G Dickson 68); K Pisi, G Pisi, Burrell (Waldouck 62), North; Myler, L Dickson (Fotuali’i 55); A Waller (E Waller 61), Hartley (c) (Haywood 61), Ma’afu (Denman 51), Lawes, Day, Clark (Dowson 54), Wood, Manoa (Dickinson 61).
Castres: Dulin; Garvey (Lakafia 68), Cabannes (c), Lamerat (Bonnefond 62), Grosso; Kirkpatrick, Tomas (Kockott 51); Forestier, Rallier (Taumopeau 51), Peikrishvili (Lazar 27), Samson, Gray, Babillot (Desroche 63), Bornman, Faasalele (Caballero 40).
Replacements not used: Coetzee.
Referee: Alain Rolland (Ireland)