The penny drops for bold Saints

Heineken Cup arithmetic is notoriously complicated – it is rumoured that Stephen Hawking took up theoretical physics only because he found the mathematics of European rugby too stiff a challenge – so it was asking a bit much of Northampton's pug-nosed front-rowers and cauliflower-faced locks to understand the importance of putting a fourth, bonus-point try past the weakened Catalans of Perpignan at Franklin's Gardens yesterday. Sure enough, it took a while for the penny to drop. Eighty three minutes and 56 seconds, to be precise.

But drop it did, and as a result of Jon Clarke's fine try in the left corner from the last play of the game, the Midlanders will travel to Limerick for Friday night's concluding pool match knowing that by taking something – anything – from the contest, they will give themselves a chance of progressing to the quarter-finals as one of two best runners-up. Victory over Munster, something of a long shot given the Irish province's astonishing record at Thomond Park, would send them through as group winners and might even sneak them a home tie.

It seemed for much of the last quarter that Northampton's director of rugby, Jim Mallinder, had used too many of his bench players too early. Lee Dickson's scrambling try just past the hour- mark signalled a touchline stampede – James Downey, Shane Geraghty, Dylan Hartley, Neil Best, the outstanding Soane Tonga'uiha and Dickson himself were all substituted within minutes of the score – and while the home side had time available to find more ways through the Catalan barricades, a flat quarter of an hour all but undid them.

It so often happens this way: the run-on team spends an hour establishing complete supremacy, only to see the hard-earned advantage frittered away by replacements who struggle to catch the tempo of proceedings. Thanks to the iniquities of the Heineken Cup fixture programme, which lumbered him with a five-day turnaround between hugely demanding matches, Mallinder had a fine judgement to make. Having gambled with his personnel, he was understandably elated to see the cards fall right. But he and everyone at Franklin's Gardens knew they could just as easily have fallen wrong.

"Had we not been faced with the Sunday-Friday problem, I would have used my bench differently," Mallinder said, tacitly admitting that with Munster on the immediate horizon, he withdrew his key players in an effort to keep them as fresh as possible. "I can't say we're happy about the fixture list. When the programme was announced we wrote a letter to the tournament administrators asking if anything could be done. Unfortunately for us, nothing has happened."

If Perpignan travelled light – most of their heavyweight players, from the backs Jerome Porical, David Marty and Maxime Mermoz to the forwards Marius Tincu, Nicolas Mas and Damien Chouly were either injured or being rested ahead of the Six Nations Championship and a major operation in defence of their French domestic title – they started in a fashion that suggested their Heineken Cup hopes were still intact, rather than dead and buried. Northampton found two forwards in particular, the strong-armed lock Yoann Vivalda and No 8 Gregory Le Corvec, difficult to subdue, and with Florian Cazanave busying himself at scrum-half, they did not deserve to reach the interval six points to the bad.

They were, however, already in trouble at the set-piece, where Tonga'uiha scrummaged Benoit Bourrust out of the game, and with their line-out under pressure – the hooker Guilham Guirardo, highly rated by the French hierarchy, missed his jumpers more than once – the Midlanders started smelling scoring opportunities straight from the restart. Chris Ashton, called into the senior England squad last week, combined with the free-running Ben Foden to send the scoreboard spinning with a try on 43 minutes, and when the same pair manufactured a score for Phil Dowson midway through the quarter, the first part of the afternoon's mission had been more or less accomplished.

"He's always there – I can't get rid of him," Foden said of Ashton. "He even hangs around my house. I need a restraining order." The last thing Northampton supporters want to see is either of these two restrained. Foden played a spectacular hand from full back, making ground with every purring foray upfield and frequently creating space with clever little passes out of contact. For his part, Ashton looked as predatory as ever. His yellowy-green boots, accurately described as "puke-coloured" by one witness yesterday, may have been an affront to good taste, but his instincts as a finisher have made him a fully-fledged member of rugby's high society.

Even Ashton has his off moments, though, and one of them might have been costly. After Hartley had messed up one chance of nailing the bonus-point try with time to spare – his cross-kick to the left corner had more than a touch of the elephantine about it – the wing was equally wasteful in toe-poking the ball into the in-goal area when it cried out for the ball to be kept in hand.

As it was, Northampton extricated themselves from this prison of their own making at the very last knockings. Both Foden and Alan Dickens played a significant role in ensuring that a sweeping attack down the left stayed alive, and when the substitute lock Christian Day appeared in space near the Perpignan 22, he maintained his cool to free Clarke on the glory run. Stephen Myler contributed a showbiz conversion from the widest possible angle, just by way of rubbing it in.

"We're attacking well at important moments, but you can't expect to win the big games with just a couple of flashy backs," Mallinder said in summary. "Winning rugby starts with ruthlessness up front." Northampton will have to be ruthless and more if they are to become only the second side to win in Limerick since the launch of the Heineken Cup in 1995, but they have at least given themselves a shot.

Northampton: Tries: Ashton, Dowson, Dickson, Clarke. Conversions: Geraghty 3, Myler. Penalties: Geraghty 2.

Northampton: B Foden; C Ashton, J Clarke, J Downey (C Mayor, 61), B Reihana; S Geraghty (S Myler, 64), L Dickson (A Dickens, 64); S Tonga'uiha (J Vickers, 65), D Hartley (capt, B Sharman, 66), B Mujati, I Fernandez Lobbe (C Day, 55), J Kruger, P Dowson, N Best (S Gray, 61), R Wilson.

Perpignan: P Burger (D Mele, 56); A Plante, C Manas, G Hume, J Michel; S Meyer (N Laharrague, 47), F Cazanave; S Chobet (J Schuster, 55), G Guirado (C Geli, 55), B Bourrust (K Pulu, 47), Y Vivalda (R Tchale Watchou, 62), R Alvarez Kairelis, B Guiry, J-P Perez (capt), G Le Corvec (Y Parent, 62).

Referee: A Rolland (Ireland).