Mark McCall's men now hold a two-point lead at the pool's summit, with Racing Metro 92 in second with 12 points and Munster a further point behind.

The first half in Watford fizzed along at an electric pace with wingers David Strettle and Doug Howlett swapping tries and Farrell and Ronan O'Gara both kicking five points.

A steady rain shower saw conditions worsen after the break and while O'Gara was able to kick Munster 13-10 ahead, Sarries' defensive control in a ferocious forwards battle was decisive as they handled anything the visitors could throw at them.

They made tackle after tackle and suffocated Munster's attacks in midfield and out wide, particularly during Will Fraser's sin-binning entering the final quarter.

By that stage, Farrell had landed a quick-fire brace of penalties as referee Jerome Garces sided with Sarries on two tight calls.

They were able to maintain that three-point buffer as O'Gara missed a drop goal and pulled a penalty wide.

Farrell had the final say for the victors late on, the returning Fraser getting the better of James Downey at a ruck and the out-half's kick sealed a six-point verdict for the English outfit.

Munster lay down an early marker in this round 4 clash with Simon Zebo soaring to gather Farrell's kick-off and James Coughlan, O'Gara and Mike Sherry combining to win a midfield turnover.

That prompted the game's first flash point with referee Garces attempting to calm matters down, with the adrenaline clearly flowing for both sides.

Saracens forced the first scoring chance, Farrell getting the hosts off the mark with a well-struck penalty in the eighth minute.

Munster prized open some space as Zebo gobbled up O'Gara's chip kick, but a free-flowing move with Casey Laulala involved twice was spoilt by a Sherry knock on.

O'Gara pushed a penalty to the right and wide on the quarter hour mark, but Munster managed to isolate kick receiver Farrell moments later and O'Gara duly levelled.

Munster had to rejig their back-line as Luke O'Dea, a late addition to the squad, was brought in to replace the injured Felix Jones with Zebo moving to full-back.

As the game entered the second quarter, Strettle dashed onto a deft Richard Wigglesworth grubber kick to score a try in the left corner with Zebo and Howlett caught flat.

Farrell flashed over the conversion, but the England international blotted his copybook when Downey picked off his high pass just inside the Munster half.

The big centre bounded away and just as he was caught, looped a perfectly timed pass out for Howlett to run in his first try of this season's Heineken Cup.

O'Gara stepped up to add the extras, with the tournament's record points scorer also finding touch with some pinpoint kicks out of hand.

Saracens were largely on the front foot through the first half and leading the way in terms of possession and territory. They were next to threaten as a Joel Tomkins break caused concern in the visitors' rearguard and McCall's side duly turned down a kickable penalty as they sensed a second try.

However, Munster showed plenty of grit at the breakdown and Laulala, who started in place of the injured Keith Earls, muscled in to win a relieving penalty in the 22.

In the closing minutes before the interval, Sarries quickly turned defence into attack to win a 34-metre penalty for hands in the ruck.

Farrell though sent the left-sided penalty wide, bringing back memories of his patchy place-kicking display from Thomond Park eight days ago.

A second penalty miss from Farrell, early in the second period and with the rain coming down, ensured the sides were still level at 10 points apiece.

Another fine high catch from Zebo should have kickstarted an attacking spell from Munster, but instead a Sarries turnover launched Brad Barritt through the middle.

There were turnovers aplenty as the rain grew heavier and the Munster pack, missing gastric bug victim David Kilcoyne, had to scrap furiously for any meaningful possession.

Coughlan, Peter O'Mahony and Donncha O'Callaghan hounded Chris Ashton into the concession of a penalty, and a surefooted strike from O'Gara lifted his team and their vocal supporters as Munster moved ahead in the 50th minute.

But the Saracens scrum, with Matt Stevens winning the plaudits, forced a scrum in front of the Munster posts six minutes later which Farrell slotted with ease.

The game was there for the taking at this point and Sarries snapped back to take a 16-13 lead as Munster infringed at a ruck following good carries from Mouritz Botha and man-of-the-match George Kruis. Farrell did the necessary from the kicking tee on the 10-metre line.

Nevertheless, the hosts soon lost Fraser to the sin-bin - for a high tackle on the advancing Howlett - and it was Munster's turn to take the ball through a punishing 22 phases in the Saracens' half.

The hard work amounted to nothing however as O'Gara flung a drop goal effort to the right and wide. Saracens had defended manfully, putting Munster bodies on the ground and maintaining their discipline at ruck after ruck.

The white-shirted visitors were a little too predictable at times and needed some variation in order to break down the aggressive defence. 14-man Sarries took obvious confidence from keeping them out.

O'Gara was off target with a 40-metre penalty, awarded after prop Stevens was pinged for not rolling away, and Munster missed out on another score - potentially a second try - when Sherry spilled the ball forward with the Saracens defence finally stretched.

With Neil de Kock finding some important touches from high pressure rucks, Saracens clawed back territory. Fraser got over the ball at a ruck after a countering Zebo was tackled, and the ensuing penalty from Farrell confirmed the home victory.