And having lost narrowly against French champions Biarritz two months ago, the Arms Park mission is crucial if Saints want to progress.

They beat Cardiff 25-11 last weekend, with Williams playing a key role against his former club, but he insists they must still improve for the trip to the Welsh capital.

“We needed a win last week, and with Biarritz losing against Ulster, that was a bonus for us,” said the Welsh international.

“It is going to be tough down in Cardiff, and we are going to have to play a lot better than we did at home against them.

“We won, but we didn’t play well. We feel that there is a lot more to offer than we showed last Saturday.”

Lions prop Tom Smith is back in the Northampton line-up after missing last week’s clash when he was dropped as a disciplinary measure for missing a team meeting.

Saints coach Wayne Smith has made one other change, calling up Spanish wing Oriol Ripol instead of Jon Sleightholme, while Cardiff, who have yet to register a win in Europe this term, are without injured Welsh internationals Rhys Williams, Martyn Williams and Andrew Lewis.

Bristol Shoguns and Swansea meet at the Memorial Stadium tomorrow, with both teams’ quarter-final hopes hanging by a thread.

With Leinster having taken a firm grip on the group, tomorrow’s winners will still realistically only have an outside chance of progressing.

Swansea, following a miserable start to the season, were greatly encouraged by their 26-19 victory over Bristol last weekend, and coach John Connolly said: “Two of our three remaining games are away from home, so it is going to be tough, but we haven’t given up the ghost.”

Bristol will hope to transfer some of their outstanding recent Zurich Premiership form on to the European stage, but a second successive loss to the Welsh outfit would spell a knockout blow.

London Irish are in a similar boat to Bristol, knowing that they must beat Madejski Stadium visitors Newport tomorrow.

Irish, who yesterday announced that their South African wing Pieter Rossouw will be leaving the club at the end of December, just six months into a two-year contract, came unstuck 16-12 at Newport last weekend.

The Black and Ambers, who produced a notable improvement following the return of Welsh stars Ian Gough and Jason Jones-Hughes after injury, are also battling to survive in Pool Five, given the French giants Toulouse are running away with it.

Toulouse are clear favourites to record a fourth successive win when they entertain Edinburgh tomorrow, while Parker Pen Challenge Cup second round, second leg action features Newcastle against Treviso and Connacht versus Narbonne.

Newcastle, still without injured captain Jonny Wilkinson, must wipe out a 19-point first leg deficit if they are to progress at the Italian team’s expense.

Falcons boss Rob Andrew has made two changes, calling up wing Joe Shaw instead of ankle injury victim Michael Stephenson and prop Marius Hurter for Mickey Ward.

Like Newcastle, Connacht also face an uphill struggle, trailing 42-27 from the first leg.