Saints had made a calm start, working through the phases patiently. The forwards were making inroads into the midfield, Vasily Artemyev was asking a lot of questions with his pace and Stephen Myler was orchestrating things nicely with both his passing and right boot.
However with just 90 seconds on the clock the visitors found themselves a man down thanks to a rare moment of Tom May indiscretion. Whatever was said between himself and Gareth Davies will probably remain on the pitch, but regardless May's decision to throw the ball at the Scarlets number nine was ill judged and referee Alain Rolland - who had received a warm welcome from the home fans - had no hesitation in sending the Saints centre to cool his heels for 10 minutes.
In his absence Saints continued to play the game at their pace and dominated possession in the opening 10 minutes, even if they did not create any scoring opportunities.
When the Welsh outfit had visited Franklin's Gardens they had capitalised on Saints mistakes. And they showed how dangerous they could be when centre Jonathan Davies hacked ahead a dropped Saints ball and put considerable pressure on Ben Foden.
Foden also showed his defensive capability to sprint 30 metres to tackle George North into touch. This stopped a probable try, but by the time May returned to the action his team was 6-0 down courtesy of a pair of Rhys Priestland penalties.
They didn't panic, however, and when Chris Ashton sent a grubber into the home 22 - following a previous burst through the midfield - he followed up his own kick to force Liam Williams to hold on and concede a penalty.
Myler made no mistake to get his team on the board, but while Saints were doing plenty of things well there was the odd thing which was letting them down and spoiling the good work.
One such instance came right at the end of the first quarter. Saints had earned a penalty for offside, but Calum Clark took umbrage at the offense and thereby gave Priestland the chance to restore the six-point lead.
Still Saints kept coming and created the first try-scoring chance three minutes later. George Pisi's strength put Saints onto the front foot and a driving maul rolled ominously towards the home line. The defence scrambled sufficiently to stop the drive just short of the line, as they did to force May into touch 10 metres out a couple of passages of play later.
The Scarlets were certainly also making the most of some interesting decisions from Mr Rolland, with one scrum in particular deep in the home 22 raising the eyebrows of the travelling faithful. Saints had got the drive on and Scarlets looked to have deliberately wheeled it. But play carried on and when a green shirt dived to secure the loose ball a penalty instead went the hosts' way.
So the hosts were able to hold out the pressure, and when they went up the other end of the field 10 minutes later they increased the lead with a Vili Iongi try. Priestland, who had created the score with a chip to the unmarked Tongan, missed the conversion, but the 11-point deficit was barely what the visitors deserved for their efforts.
Myler pulled three points back almost immediately, Artemyev setting up the chance by winning the kick off back. But while this put Saints back in touch they still suffered a massive scare just before the break when North chose to pass close to the visitors' line instead of pinning his ears back.
Saints needed a blistering start to the second half, and with a Pisi interception - closely followed by a great Foden break - they put the Scarlets onto the back foot. But it was another driving maul that prompted referee Rolland into calling TMO Seamus Flannery into action for the first time.
It had started 10 metres out from a lineout, and while there was no conclusive evidence that Clark had touched the ball down Saints still had the scrum. The first motored towards the line but was called back. The second saw the Scarlets edge the hit, but Artemyev showed his awareness to come off the wing and Soane Tonga'uiha stretched over the score.
It was just what the Saints were looking for, and when Myler landed his third penalty with half-an-hour to go he nudged the visitors in front for the first time. Priestland replied quickly, but Myler was on song from the tee and two more penalties edged Saints out to a five-point lead as the final quarter approached.
The visitors had had all of the play thus far in the second half, and had had their reward, but as the clock ticked past the hour they found themselves having to defend a Scarlets team bolstered by the introduction of Welsh legend Stephen Jones.
But Jim Mallinder had reinforced, too, with Lee Dickson, Paul Doran Jones and Christian Day all seeing action. And they helped their team not just hold out some ferocious Scarlets attacking but also to come within inches of killing the game off with eight minutes to go.
Having forced Ben Morgan off his own ball Roger Wilson powered towards the line. One green shirt after another tried for the whitewash and the march seemed inexorable. But Artemyev had the ball stripped by Williams, and this inspired the Scarlets to go upfield and set up camp at the other end of the field.
Cue some frantic drives for the line and some even more desperate tackling from the men in green. One massive tackle by Dickson on Deacon Manu stood out, but half a dozen times a Scarlet looked like scoring only to be stopped in his tracks by an opposite number.
A turnover helped relieve the pressure a little, but back came the Scarlets with Williams forcing Foden to pull another outstanding tackle out of the bag to prevent a certain score.
The stage was set for a thrilling final few minutes, with the Scarlets pushing and pushing towards the line. Then just as it looked like the travellers were on the verge of suffering yet another European heartbreak up stepped Foden with an interception and 95-metre run that sealed the game and meant that Saints will run out at a packed stadiumMK next week with plenty to play for.
REFEREE Alain ROLLAND
2mins SAINTS Yellow card May
8mins SCARLETS Pen Priestland 3-0
13mins SCARLETS Pen Priestland 6-0
17mins SAINTS Pen Myler 6-3
19mins SCARLETS Pen Priestland 9-3
33mins SCARLETS Try Iongi 14-3
35mins SAINTS Pen Myler 14-6
HALF-TIME SCARLETS 14 SAINTS 6
45mins SAINTS Try Tonga'uiha Con Myler 14-13
50mins SAINTS Pen Myler 14-16
52mins SCARLETS Pen Priestland 17-16
54mins SAINTS Pen Myler 17-19
57mins SAINTS Pen Myler 17-22
78mins SAINTS Try Foden Con Lamb 17-29
FULL-TIME SCARLETS 17 SAINTS 29