Castres had been one of the most physical sides the Saints had faced in 2010/11, and they lived up to their reputation with a game plan with little subtlety, teeing up their big runners to get them over the gain line.
Chris Masoe and Rodrigo Capo Ortega got their hands on the ball on behalf of the forwards, while Seremaia Bai and Romain Martial were Castres' go-to men out wide. And when the tactics paid off inside two minutes Saints were under no illusions that they were in for a big contest.
Saints had started the game well, Vasily Artemyev winning back the kick off and George Pisi combining with Ben Foden down the right wing. But a penalty ended the move, starting a train of events which led to a try for Steve Malonga. The flanker had got his team on the front foot with a powerful charge and was on hand when Martial gave him a neat offload close to the line.
Romain Teulet lived up to his metronomic reputation by slotting the conversion from the touchline, the first of five successful kicks at goal from the full back. But while Teulet kept the scoreboard ticking over in the first half it was his schoolboy error which ultimately led to Saints' opening try.
In letting a Stephen Myler bomb bounce he let Jamie Elliott apply a lot of pressure, and while Castres won a penalty the quick tap was fumbled and Saints had a scrum in the 22. This became a penalty and a kick to the corner, and from the lineout Mark Sorenson was eventually driven over.
In outstanding conditions for running rugby both teams had their chances. Castres came close when Masoe was stopped in his tracks by Samu Manoa, but Saints would have been kicking themselves when a great opportunity went begging with 25 minutes on the clock.
Playing with the advantage Myler spread the ball wide to try and take advantage of an overlap. But for some reason Ben Foden cut back inside and spilt the ball, and although play came back for Myler to kick to the corner the forwards narrowly failed to maintain control on the line and Castres cleared.
This was compounded by Teulet's second penalty, but Saints turned threat into points within 60 seconds. Ibrahim Diarra tapped the restart straight to Artemyev, and with Pisi making massive inroads into the home midfield an overlap was created and finished off by Elliott.
Teulet replied with a penalty but Castres ended the half as they had began it, with a try in the right corner. Myler had seen yellow for a high tackle on Thomas Sanchou - a decision which seemed at tad harsh given the diminutive centre looked to have slid into the challenge - but it was the fly half's second such offence and referee John Lacey had no hesitation in reaching for his pocket.
Pierre Bernard sent the ball to the corner, and although Saints defended manfully the personnel ran out and Masoe slid through a kick for Romain Martial to touch down.
Teulet's conversion made the score 23-12 going into the break and it was imperative that the visitors scored first following the resumption of play.
This they duly did, but ironically enough it looked like Saints had butchered their opportunity when Ben Nutley forced a pass. But when Castres pressed Elliott backed himself and juggled the interception before sprinting 60 metres for the try. May's conversion went agonisingly close to the right-hand upright but nevertheless Saints had had exactly what they needed.
Teulet maintained his hundred percent record five minutes later, but Saints had noticeably upped their tempo and concentration and when Manoa powered over from close range in the right corner they had a bonus point to their name and 25 minutes to claw the final few points back.
The May/Pisi/Foden/Elliott combination was clicking superbly and causing palpitations among the home fans whenever they got the ball. But despite stretching Castres the final man seemed to just be there in time to bring the attack to an end.
But then all of a sudden Tom Wood was yellow carded for offside at a maul and just as it had in the first half momentum swung the hosts' way once again. Teulet kicked the subsequent penalty to stretch the lead to seven, but when Martial made the most of the extra space with Wood still off the field to score his second try it all but secured the win for the Tarn outfit.
Saints' efforts didn't waver, Artemyev and Pisi both making dangerous breaks. Ryan Lamb was looking lively, too, but it was increasingly in vain and Ibrahim Diarra rounded things off with a try in the left corner.
REFEREE John LACEY
2mins CO Try Malonga Con Teulet 7-0
9mins SAINTS Try Sorenson Con Myler 7-7
17mins CO Pen Teulet 10-7
30mins CO Pen Teulet 13-7
31mins SAINTS Try Elliott 13-12
34mins CO Pen Teulet 16-12
38mins SAINTS Yellow card Myler
40mins CO Try Martial Con Teulet 23-12
HALF-TIME CASTRES 23 SAINTS 12
44mins SAINTS Try Elliott 23-17
49mins CO Pen Teulet 26-17
55mins SAINTS Try Manoa 26-22
62mins SAINTS Yellow card Wood
63mins CO Pen Teulet 29-22
68mins CO Try Martial Con Teulet 36-22
77mins CO Try Diarra 41-22
FULL-TIME CASTRES 41 SAINTS 22