The home side ran in three tries at Parc y Scarlets as they made amends for their defeat away to the same opposition last weekend.
And they caught their opponents off guard from the from the first whistle, Nic Reynolds slicing through the Scottish defence to touch down.
Things then got better for Llanelli when Diggy Bird extended their lead with a penalty before scrum-half Gareth Davies added a simple try, converted by Bird.
The home side were at their best and added another try after just 14 minutes when full-back Dion Jones chipped ahead and recovered the ball brilliantly to score.
Centre Oli Grove got the visitors back into things with a converted try and he then added his second score in response to another penalty from Bird.
In a dramatic second half, each side had chances to seal the win as both kickers exchanged shots at goal. But it was Llanelli who were to grind out the win, a penalty from replacement fly-half Owen Williams ending Scottish’s hopes.
In Pool 3, Carmarthen Quins’ hopes of progressing to the quarter-finals were hit by a 25-23 defeat away to Dundee.
After a win over the Scottish side last weekend, Quins travelled north in confident mood but were edged out in a narrow defeat. James Dixon was among the scorers for Quins as they were outscored three tries to two by the Scottish side.
Quins’ defeat means Cornish Pirates are now firm favourites to progress from that pool and they strengthened their position with a 10-30 win over Swansea at St Helens.
The English side led narrowly 7-10 at half time but pulled clear in the second half.
Both Bridgend and Cardiff faced tough clashes with Bristol and Ulster respectively while Newport were on the receiving end of a hungry Newcastle at Kingston Park.
Elsewhere, Neath and Cross Keys put in credible performances on their travels which bode well for their return to Principality Premiership action over Christmas.
Dorian Jones went over and his namesake Steffan kicked the rest of the points but it was not enough to prevent Keys slipping to a 31-16 defeat away to Connacht.
Neath lost out to Bedford while in Pool 7, Aberavon were beaten away at Plymouth Albion.
