Northampton trio Paul Diggin, Ben Foden and Jon Clarke lit up a dour match with excellent tries as Saints maintained their unbeaten Guinness Premiership home record to add to bottom club Bristol's relegation worries.

It was the fourth Saints victory over the west country side this season following two wins in the European Challenge Cup and one in the EDF Energy Cup.

But the result was a major setback for Bristol who had targeted the game for some much-needed points after winning their home Premiership clash between the two sides.

With fly-half Stephen Myler kicking their other 15 points, victory prevented Northampton, whose away form has not matched their performances at home, from being dragged into relegation trouble.

Their chairman Keith Barwell had labelled the fixture "a must-win match" and the pressure on both clubs was evident in a nervy, disjointed first half where neither side really threatened to score a try until Diggin struck.

Myler missed a straightforward penalty for Northampton in the fourth minute and Barnes was wide with a long-range kick for Bristol in the 21st.

Former England scrum-half Shaun Perry, skippering the visitors in the absence of the injured Joe El Abd, and fly-half Ed Barnes worked hard to try and get their side moving but it was Northampton who struck the first blow in the 29th minute.

Hooker Dylan Hartley, watched by England scrum coach Graham Rowntree, had a try disallowed when referee Tim Wigglesworth brought him back for a disputed earlier knock-on.

But Bristol were penalised at the resulting scrum and Myler kicked Northampton ahead with a more difficult penalty than the earlier one he had missed.

Three minutes later Diggin produced an excellent burst of speed to exploit weaknesses in the Bristol defence and score the opening try for Northampton.

Scrum-half Lee Dickson fed Foden, preferred at full-back with club captain Bruce Reihana consigned to the bench, showed fast hands to release Diggin who showed a clean pair of heels on his 50 metre dash to the line after

centre Luke Eves missed his tackle.

Myler increased the lead to 13-0 with a 37th minute after Perry, going for an ambitious interception, was somewhat harshly punished for a deliberate knock-on.

Hartley, up against England's 2007 World Cup hooker Mark Regan, did his international prospects no harm as he attempts a Six Nations challenge to Bath's current England No 1 Lee Mears, who was preferred ahead of the Northampton man for the autumn internationals.

Myler's third penalty, in the 48th minute, put Northampton 16 points clear.

Bristol were caught offside when Scotland winger Sean Lamont took a tap penalty after being the victim of a high tackle by former Wales second row man Robert Sidoli and the former rugby league fly-half confidently booted over the kick from 40-plus metres.

The latest blow seemed to spark the visitors to life and the pressure paid off when Saints conceded a penalty in front of their own posts and Barnes finally opened Bristol's account in the 50th minute.

They spurned a penalty opportunity in favour of a lineout in the corner before replacement Nathan Brew - brother of Neil - was held up inches short of the line but an infringement at the resulting ruck cost Bristol their momentum.

Saints retaliated to wrap up the match when Foden broke three tackles on a tremendous 40-metre diagonal run through the Bristol defence to touch down near the posts in the 73rd minute, with Myler converting.

They struck again in the final minute when poor defending allowed centre Clarke in for their third try, with Myler again adding the kick but, with seconds remaining, always-lively left-wing David Lemi kicked ahead and won the chase for a consolation try.