Jason Leonard lifted the Parker Pen Challenge Cup on his final Harlequins appearance after Montferrand pressed the self-destruct button.

Quins secured their place in next season’s Heineken Cup through a sensational try deep into injury time by substitute Keogh, which his fellow replacement Dunne converted to spark scenes of wild celebration.

Quins were outplayed for much of the match, but Montferrand had centre Raphael Chanal sent off in the 78th minute and the 14 men he left behind could not hang on.

Full-back Gavin Duffy scored a first-half try for the Londoners, while fly-half Paul Burke kicked four penalties and Dunne also slotted a penalty.

Montferrand, whose Heineken Cup hopes disappeared with defeat, claimed touchdowns from man of the match Olivier Azam and scrum-half Pierre Mignoni.

Full-back Anthony Floch booted 13 points and fly-half Gerald Merceron dropped a goal, but it will remain a mystery of Agatha Christie proportions how Montferrand contrived to lose a game they so clearly dominated.

The lead changed hands on several occasions, keeping a 13,000-plus crowd enthralled, yet Quins struck when it mattered most, capitalising on space out wide created by the reckless Chanal’s departure.

And it gave Leonard, rugby’s world record cap holder, a glorious send-off after he went on as a substitute seven minutes from time.

Montferrand, whose Heineken Cup status next term depended exclusively on victory, made the early running, but Merceron wasted a glorious try-scoring chance after four minutes when he ignored a one-man overlap and Quins cleared the danger.

Floch then sent an angled penalty wide of the posts, and Quins were relieved to still be on level terms, given Montferrand’s relentless pressure.

And the escape act was completed on 12 minutes when Burke bisected the posts with his first penalty, edging Quins 3-0 ahead.

Floch slotted an equalising kick three minutes later, but Quins now had the bit between their teeth and they unlocked Montferrand’s defence in simple fashion.

England star Will Greenwood’s cleverly-placed kick drifted behind retreating Montferrand wing Sebastien Kuzbik, and, as he tried to recover possession, Duffy nipped in and punished his hesitancy.

Montferrand had to reply swiftly, and Quins were stretched to keep out charging wing Aurelien Rougerie before the visiting forwards piled in and Azam was awarded the try after referee Nigel Whitehouse consulted video official Nigel Williams.

Floch converted, putting Quins 10-8 behind, only for Montferrand to have flanker Marc Raynaud sin-binned following a technical offence.

Burke arrowed the resulting penalty wide, but he should never have had the opportunity as Quins scrum-half Scott Bemand took a quick tap and run before Whitehouse offered them a second chance.

Montferrand though, were undeterred and Merceron landed a drop-goal from 35 metres before Burke’s second penalty kept the Londoners in touch.

He completed his hat-trick four minutes before half-time – with Montferrand conceding an extra 10 metres because of dissent – but Floch responded with his second penalty, sending Quins in 16-14 behind at the break.

As in the first half, Montferrand made all the initial running, using flanker Olivier Magne in a more prominent ball-carrying capacity, but a penalty conceded on halfway allowed Quins to gain territory.

It was tense and edgy, with both sides making handling errors in perfect playing conditions, and Burke’s 30-metre penalty saw the lead change hands for a fourth time.

Quins suffered a 55th-minute injury blow when centre Mel Deane was felled after he attempted to tackle wing Aurelien Rougerie.

Ultimate hero Keogh replaced him, and Montferrand kept up the pressure as the third quarter neared its conclusion.

Montferrand finally broke through on 58 minutes. Again their forwards smashed through the hub of Quins’ defence, and Mignoni claimed the touchdown.

Floch mocked his earlier misses by landing the touchline conversion, and there was a degree of daylight between the teams with Montferrand 23-17 ahead.

Burke missed a straightforward 59th-minute penalty, and it was to be his last contribution before he limped away from the action and was replaced by Dunne.

Quins boss Mark Evans also sent on prop Ceri Jones for Mike Worsley, and Dunne’s first contribution was to kick a penalty after Chanal was sin-binned for punching.

Raynaud strolled through Quins’ defence after gathering a loose ball from a line-out, but Whitehouse called him back, having already given Montferrand a penalty and not playing advantage.

Floch secured the three points though, putting Montferrand 26-20 ahead and setting up an exciting finale.

Leonard entered the action to a standing ovation on 73 minutes, but Montferrand still had time to capitulate.

Chanal was sent off after receiving a second yellow card for punching, and Quins threw everything at them.

The winning try duly arrived through Keogh, then converted by Dunne, sending Quins into top-flight European competition next term and Montferrand to wonder how on earth they lost.