They were not given much of a chance – and sure enough, for most of the match they did not have one. But Northampton, having looked completely lost in front of the 82,000 spectators at Croke Park, burst into life in the final quarter to give Leinster something of a scare.

The home side, though, are too packed to the rafters with international quality to have lost from such a position. They have had their wobbles in the recent past, but letting slip a 20-3 lead early in the first half would really have been too painful to contemplate. As it is, with James Lowe – one of the very best players in the Six Nations, let alone the Champions Cup – bagging a hat-trick, they march on to London for the final at the end of the month.

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“We made a lot of mistakes,” said Courtney Lawes, Northampton’s captain, “and you can’t win at the highest level if you do that. But on the plus side we were able to push one of the best teams in the world.”

One of the best teams or not, Leinster are not infallible. A little less than a year ago the other side of the river, Leinster galloped off to a 17-0 lead against La Rochelle in last season’s final at the Aviva Stadium, only to lose. They did not quite start as riotously as that, but a 12-0 lead in the first quarter of an hour posed Northampton a big problem.

The Saints were as complicit in the problem-setting as Leinster were proactive. Both positions from which Lowe scored his two early tries came courtesy of Northampton errors.

Gloucester reached the Challenge Cup final for a fourth time after beating Italian hopefuls Benetton 40-23 at Kingsholm. George Skivington’s team will face South Africa's Sharks at Tottenham Hotspur Stadium on 24 May. Despite struggling in the Premiership this season, Gloucester are now chasing more silverware after being crowned Premiership Rugby Cup winners in March.

The fly-half, Adam Hastings (pictured), was the architect of Gloucester’s triumph, scoring 17 points through a try, three conversions and two penalties on his penultimate home appearance before rejoining Glasgow this summer.

The full-back Josh Hathaway, hooker Seb Blake, lock Freddie Clarke and wing Ollie Thorley also touched down, while the scrum-half Caolan Englefield kicked a penalty 10 minutes from time.

Benetton, Challenge Cup semi-finalists last term, pushed Gloucester hard as hooker Gianmarco Lucchesi (two) and full-back Rhyno Smith scored tries - Tomas Albornoz added two penalties and a conversion – but two tries in four minutes midway through the second period gave the home side breathing space.

The Sharks booked their place in the final after striking with a late try to topple Clermont 32-31 at the Twickenham Stoop. The 71st-minute score was made possible by Eben Etzebeth successfully challenging for a kick, with the South African side's attack then clicking into gear. A superb offload from Vincent Tshituka found Lukhanyo Am and he drew the last defender before providing his  Springboks team-mate Makazole Mapimpi with a simple run in.
The Sharks, who were 31-18 down heading into the final quarter, had Aphelele Fassi in the sin bin when Mapimpi pounced and still needed Siya Masuku to nail a tricky conversion to win. PA Media

James Ramm fumbled deep in the corner, and Leinster set up camp from the resultant scrum. Northampton conceded a penalty during the siege, which Jamison Gibson-Park tapped. He flung a loopy pass to Lowe out wide, who ran clean through Alex Mitchell and Ramm for the opening try.

That was after 10 minutes of play. His second came within five of that. This time Fin Smith was guilty of a careless error, his pass intercepted by Ross Byrne to set up more pressure and force the Saints into the concession of another penalty. Byrne went for the corner, Caelan Doris peeled off the driven lineout to go for the line and from his flip-up Gibson-Park managed to fashion another pass out wide to Lowe, who scored at his leisure.

Northampton looked lost. Handling errors plagued their every move. Not until a minute from the break did we see anything of the slick attacking with which they have lit up the Premiership.

By that time a Byrne penalty on the half-hour had furthered their pain, Trevor Davison deemed to have collapsed a scrum in front of Northampton’s own posts, but a smooth move in midfield might have put Ramm away. Alas, he overran George Furbank inside him and could not quite take in his pass. Still, Leinster had conceded a penalty themselves, and Smith slotted it to put the visitors on the board.

Any respite was temporary. Lowe had his hat-trick three minutes into the second half. The calibre of player coming at Northampton from all angles was of international standard. Ryan Baird was the next to show off, galloping clear on the counterattack. Mitchell managed to fell him, but then it was Robbie Henshaw down Northampton’s left, then Dan Sheehan feeding Ciarán Frawley on the loop. His pass put Lowe into the corner for a 20-3 lead.

That made the game feel safe. And matters duly meandered, the record crowd only sporadically rousing themselves to celebrate the latest overpowering of a visiting ball-carrier or defender.

On the hour, though, Northampton struck at last. They spoiled a Leinster lineout and were on the attack. Alex Moon’s long pass found George Hendy on the left. His chip and chase was missed by Jordan Larmour coming across, leaving Hendy free to dot down. Smith’s conversion from out wide pulled Northampton back to within 10.

The first hints of Leinster nerves may have revealed themselves when Byrne pushed a penalty wide a couple of minutes later, but it was not until the last 10 minutes that the game suddenly came alive. Gibson-Park was forced into carrying the ball over his try-line, and the Saints had their foothold. Elliot Millar-Mills charged close, before Fraser Dingwall’s long pass found Tom Seabrook unmarked on the left. Smith’s conversion had Saints within three points with five minutes to play.

It looked, thrillingly, as if the Saints might have carved out an opening out wide, but Leinster’s cover was good. When Jack Conan swooped at a tackle to win the penalty, Leinster finished the game in control. Never in doubt.