In his post-match comments director of rugby Jim Mallinder said he had wanted "a step up" from the Round 3 performance at the Stade Pierre Antoine.
But while Saints eventually turned on the style to score all five of their tries in just 18 minutes, the one-way traffic had looked a distant dream following a first half which had featured just the one successful kick at goal from Stephen Myler and precious few sights of the try line.
Alex Waller had come closest to scoring, his 23rd minute reach for the line being ruled out by the TMO for fumbling the ball. This in turn had followed a yellow card shown to Castres flanker Pierre Roussel for barging Myler out of the way while the fly half was chasing his own kick to the line, and given how the hosts were bossing things in the scrum and trying their hardest to maintain a brisk tempo it seemed only a matter of time before they would get on the scoreboard.
But Castres, who seemed intent on slowing the game down at every opportunity, somehow managed to step up their own pace in defence whenever the Saints got in the danger zone.
This happened regularly in the first half. Vasily Artemyev was the main runner, popping up across the field, with Paul Doran Jones and Courtney Lawes two forwards to show impressive turns of pace in breaking the initial line of defence. Mark Sorenson came within a couple of metres of the line but here, as with so many opportunities in the first half, somehow a Castres shirt arrived in the nick of time to break up the attack.
Despite having made 13 changes from the team which had won in France last weekend the visitors looked off the pace, only threatening when Pierre-Manual Garcia chipped the rushing defence and collected the bouncing ball.
The first half was typified in the last play of the half. Lawes did all the hard work, breaking through the initial tackle and stretching his legs into space. But the last tackle from Florian Denos forced the ball from the big lock's grasp and with the fumble the teams headed into the changing rooms.
When the kick off was won but then fumbled it looked like Saints were going to have the same frustrations as they'd had before the break. But a swift trio of penalties from Myler gave the hosts a bit breathing space, and when Pierre Bernard - who had missed a first half shot at goal - had his second attempt at the sticks drop short it provided the cue for the Saints to seal the game.
The 11,670-strong crowd got the try they craved just past the hour mark. Jamie Elliott cut in off his wing to take Lee Dickson's inside pass, and when the ball was recycled quickly Ben Foden was over unopposed.
The second score, just six minutes later, was an absolute peach and a candidate for try of the season. Jon Clarke may have juggled the clearance kick on his own 10-metre line, but there was no doubting his pace and strength that took him well into the Castres half. Once again quick ball proved to be the key and with the visitors retreating some great hands from Mike Haywood, James Craig and man of the match Samu Manoa put Artemyev away and over the line.
Next up it was forwards who got in on the action, driving Haywood over for his first Saints try. And with Myler maintaining his 100 percent kicking record off the tee the contest was over at 33-0.
There was now the small matter of a bonus point to aim for. But while Scott Armstrong and Jamie Elliott combined superbly to put the Yorkshireman into the 22 the first half fumbles had started to creep back into the Saints' play and the fourth try was playing hard to get.
Having flirted with the whitewash the Saints finally got over to earn maximum match points when Greig Tonks left the defence for dead to dot down unopposed. And appropriately enough it fell to Foden to round things off with a 60-metre special, breaking clear from his own 10 metre line and turning on the afterburners to leave the defence for dead.
REFEREE George CLANCY (IRFU)
8mins CO Miss pen Bernard
12mins SAINTS Pen Myler 3-0
22mins CO Yellow card Roussel
HALF-TIME SAINTS 3 CASTRES 0
44mins SAINTS Pen Myler 6-0
47mins SAINTS Pen Myler 9-0
51mins SAINTS Pen Myler 12-0
59mins CO Miss pen Bernard
61mins SAINTS Try Foden Con Myler 19-0
67mins SAINTS Try Artemyev Con Myler 26-0
70mins SAINTS Try Haywood Con Myler 33-0
77mins SAINTS Try Tonks 38-0
79mins SAINTS Try Foden Con Myler 45-0
FULL-TIME SAINTS 45 CASTRES 0