The Welsh region have now won four from four this season to top the table and it was their Canadian flyer - so impressive in the World Cup and playing his first game since moving from Glasgow Warriors - who caught the eye.

James Davies scored the Scarlets first try before van der Merwe bagged a try in each half, Steven Shingler adding ten points from the kicking tee.

While the Scarlets top the table, depleted Leinster have won two and lost two but they will take some heart from Isa Nacewa and James Tracy's late scores as they finished the game in the ascendancy.

Coming into the game on a seven-match winning streak, it was no surprise that the Scarlets began the game with confidence and in the fourth minute Shingler kicked them into a 3-0 lead after Leinster failed to roll away.

With so many Leinster players still with Ireland at the World Cup, this was another chance for their young stars to impress and Noel Reid's early break and chip through brought good territory.

But the chance came to nothing and up the other end the Welsh region were soon further in front, wide-to-wide attack stressing the Leinster defence and creating an overlap that Davies joyously accepted, the flanker sliding over for the game's first try.

Shingler missed with the conversion but up the other end Fergus McFadden's first penalty attempt fell short so the hosts remained 8-0 ahead.

Nacewa was then bundled into touch by a superb covering tackle from Hadleigh Parkes to deny Leinster their first try as they ramped up the pressure on the half hour mark.

A van der Merwe knock on when the Scarlets tried to play out of their own 22 then ramped up the pressure and when Garry Ringrose stepped and released McFadden a try looked on the cards. But the ball was spread right and van der Merwe - fresh from four tries for Canada in the World Cup - made amends for the knock on to pick off Reid's pass and go the distance for the score.

Shingler's conversion made it 15-0 and that was how it stayed until the interval.

After the interval the Scarlets emerged on fire, breaking the line at will and Reid was soon in the sin bin for Leinster, the centre punished for repeated team infringements.

Still the red wave of Scarlets attacks kept coming, Regan King putting his backs into space seemingly at will and eventually the dam broke when van der Merwe scampered over from short range down the left for his second and the team's third try.

Shingler added the extras for a 22-0 lead but Reid's return from the sin bin sparked Leinster back to life, the centre releasing Nacewa for their first try of the night.

McFadden added the extras and five minutes later he should have added a try but knocked Cathal Marsh's pass on with the try line begging.

Late on Tracy did manage to barge over at the back of a maul for Leinster's second but it was too little too late at Parc y Scarlets.

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