With Castres Olympique falling 18-12 to Northampton Saints in Pool 4's other fixture, Ulster are now guaranteed a place in the last-eight - although a home quarter-final is still to play for in next Saturday's clash with Castres in the south of France.

Mark Anscombe's side was bolstered by the returning Payne at full-back, while Paddy Wallace reclaimed the number 12 jersey after the injury sustained by Luke Marshall in last Friday's league victory over the Scarlets.

Three further changes among the forwards saw Rory Best reinstated at hooker, John Afoa start at tighthead prop and Iain Henderson claim his 11th cap in the second row with Dan Tuohy sidelined due to a calf strain.

After a disjointed opening five minutes punctuated by several knock ons of a slippery ball by the Warriors, Pienaar opened the Ulster account with a penalty from outside the 22 - just before the rainfall escalated to a torrential level.

The increasingly difficult playing conditions soon ended any hopes either team had of producing truly constructive rugby at this stage, although a smart kick from Cave into the corner after 18 minutes almost set Andrew Trimble away before Peter Murchie scampered back to kick into touch.

Ulster worked the lineout well, powerfully driving ever closer to the line until crowd favourite Williams took over with a metre to go and barged over for his third try in as many games. Pienaar added the extras for a 10-0 scoreline at the end of the first quarter.

A second Pienaar penalty attempt spun wide five minutes later, and the very next move almost led to a second home try, as Glasgow inexplicably surrendered the ball under no real pressure in their own 22.

But here, once again, the greasy ball undid the attack as Ulster fumbled on the right wing. The half then closed on two errant Duncan Weir penalty attempts, ensuring Ulster went in at the break with their ten-point buffer intact.

With the rain at a more manageable level as the second period began, Glasgow started to edge their way into the encounter, and were well worth the three points struck by Weir's boot in the 48th minute.

The Scots continued to enjoy the lion's share of possession and with Henderson sin-binned for a ruck offence after a succession of Ulster transgressions, out-half Weir nudged over a close range penalty to bring his team within touching distance.

Paul Marshall and Roger Wilson made their entrance on the hour mark in lieu of Paddy Jackson and Robbie Diack respectively, Pienaar and Williams moving to out-half and blindside flanker respectively as a result.

The Springbok restored Ulster's seven-point cushion after 62 minutes courtesy of a central penalty. He only just failed to widen the gap moments later after Glasgow had been pulled up for infringing at the scrum.

It mattered little, however, as the definitive breakthrough came on 73 minutes, Payne touching down in the right hand corner after Ulster's most fluid collective move of the match.

There was good approach work from Marshall, in particular, to extract the ball from several congested mauls before spreading play wide. Pienaar was unlucky to see his conversion attempt rebound off the upright from a tight angle.

The highlight of the match came with two minutes remaining when, with the contest won and many of the 10,940-strong crowd already making their way to the exits, Cave capped a strong individual display with a memorable solo try.

Picking up on the left wing with a lot of work to do, the classy centre danced his way past a handful of lunging challenges before cutting inside to evade several more defenders and slide over the whitewash under a final despairing tackle.

Pienaar converted and although Ulster made a last-ditch attempt to grab a bonus point right at the death, they ran out of time and had to settle for a four-point return.

The sight of bulldozing back rower Williams hobbling off with a knee injury will have concerned the province's coaching staff as they contemplate the round 6 trip south.

With one round of pool matches remaining, Ulster now sit on 19 points - five clear of Saints and six ahead of next weekend's opponents Castres. A home quarter-final is just 80 minutes away.