Saracens, whose European ambitions were twice frustrated by Munster last season, belatedly solved the Irish question in Watford yesterday. But it took much scratching of heads before the 1999 Heineken Cup winners were buried beneath an avalanche of points in the final quarter.

With an hour gone and Ulster 25-24 ahead, Saracens were gasping for air. It was then that Thomas Castaignede helped stunningly to transform the game.

The Frenchman's blonde hair may come from a bottle but his Rugby skills are the genuine article.

On the hour his brilliant run from the half way line gave Dan Luger a first decent pass of the afternoon. The wing gratefully snapped it up but was stopped short of the line only to set up a ruck for Duncan McRae to wriggle over for a try that began the deluge of 31 points without reply.

Castaignede has now landed 183 points, making him by far the highest scorer and Saracens have already notched up four half centuries this season.

The lopsided final score may suggest an easy victory. It was anything but. From the moment Ulster's prop Justin Fitzpatrick flattened Kyran Bracken straight from the kick-off, Saracens were given many painful reminders of the robustious side of Irish rugby.

David Humphreys, the Ulster captain in his familiar scrum-cap, had Saracens constantly on the back foot with his astute tactical kicking in the first half and with the English club also frustrated by the Italian referee Signor Giaconel, in a very generous mood towards the visitors, the spectre of Munster loomed.

Richard Hill, at standing on the Saracens flank, crashed over for two identical tries on either side of the interval but as Saracens consistently erred, the referee constantly penalised them for every offence in his book and Humphreys constantly accepted the points on offer.

Saracens were then stunned just after the break when the Ulster pack drove forward from a line-out and presented quick ball for Brad Free and Humphreys to give their centre Shane Stewart an opening. Stewart had quite some work to do but blasted through the Saracens midfield wall with ominous ease.

Castaignede and Humphreys continued to trade penalties. Saracens then turned over possession and Castaignede broke from his own 22, slipping into over drive and running 60 metres before off loading to McRae. The Australian gave Humphreys a hand off that wouldn't have disgraced Lennox Lewis and Luger scored near the posts.

Sniffing blood Saracens went for the kill. Luger scored a second try after sustained pressure and completed his second hat-trick of tries this season with the last move of the game.

He broke from deep inside his own half after Saracens had won a scrum and moved the ball to the blind side where Luger's touch line run outstripped what remained of the Ulster cover.

It has been a mixed weekend to say the least for English clubs but Saracens look by far the best equipped to repeat Northampton's triumph in this competition. Even when they are struggling to play in the up tempo style they enjoy and even missing vital forwards like Julian White and Scott Murray they seem to be able to raise their game.

White may be back in contention against Cardiff next week, the England prop making a swift recovery from his shoulder operation. But Francois Pienaar, the Saracens chief executive whose left arm is in a sling after his own shoulder operation echoed his captain's thoughts.

"That was the worst first half we'd played this season. Their defence really rocked us back. Having said that some of our tries were fantastic but Cardiff will be a do or die game and we need to relax a bit more."

Saracens supporters, and there were 11,258 here yesterday, can relax, though. A quarter-final place surely awaits after that awesome 20 minutes.

Saracens: Castaignede; O'Mahony (Sparg, 70 min), Johnston, Sorrell, Luger; McRae, Bracken (capt, Walsh, 70); Flatman, Russell (Cairns, 70), Wallace (Philips, 77), Davison, Grewcock, Chesney, Hill, Diprose.

Ulster: Henderson; J Topping, Constable, Stewart (Coulter, 79), Howe; Humphreys (capt), Free; Fitzpatrick, Weir, Best, Johns (Blair, halftime), Longwell, D Topping (McCarey, 68), Ward, McWhirter.

Referee: C Giaconel (Italy).