Following the agonising 13-12 loss to Argentina in Wellington, Andy Robinson's men are battling to progress from Pool B to ensure they do not become the first Scotland team to return from a World Cup without progressing to the knockout stages.
Scotland and Argentina, who are fancied to beat Georgia on Sunday, are tied on 10 points, with England currently leading the group on 14.
Scotland must win at Eden Park on Saturday and by a margin of eight points - otherwise England would claim a losing bonus point and progress.
As the post-mortem to the loss at Wellington Regional Stadium took place today before the team's departure for Auckland tomorrow, Barclay attempted to accentuate the positive after Scotland missed their first chance to advance to the last eight.
The 25-year-old Glasgow Warriors openside said: "It's not about making massive changes - we're not going to reinvent the wheel in the space of a week.
"It's just about looking at the game, looking at what we could've done better - the same as every week - and going out with belief and confidence that we can go out there and win and win well.
"We've been building towards this World Cup for months now.
"We realise if we lose we go home - there's no bigger stage and no bigger game."
Against Argentina, Scotland surrendered a six-point lead with eight minutes remaining as Lucas Gonzalez Amorosino touched down following a mistake-riddled period and Felipe Contepomi kicked the touchline conversion.
The opportunity to win was then missed one minute from time.
Dan Parks kicked a drop goal wide under pressure from the Pumas defence, who, unbeknown to the officials, appeared to have strayed offside, and Scotland suffered defeat for the first time in five matches.
Barclay added: "I felt we were in control the whole game and one 30-second lapse after going six-points clear... that's why it's so frustrating.
"I still thought we'd win the game (after the converted try). "We got into a great position, won the line-out, a couple of phases and we passed it out too early.
"Ideally we'd have kept it going until we were right in front of the posts, but something happened and the ball came out.
"That's hindsight and that's pressure and that's what happens."
Barclay summed up the fine margins in top-level sport - had Parks' drop goal been successful, Scotland would have been into the last eight with one match to play.
He said: "We put ourselves in a position and if we got the drop goal at the end everyone would be saying it's great.
"It's a difference of 10 seconds or five feet, however you want to put it."
Most of the discussion in the Scotland camp since the heart-wrenching loss has been about the Pumas game, but now attention turns to England.
Scotland last beat England by eight or more points in 1986.
In the 25 years since, Scotland have won four times - in 1990, 2000, 2006 and 2008 - all by a six-point margin and all at Murrayfield.
But Barclay and his team-mates will take confidence from the 15-all draw in Edinburgh during the 2010 Six Nations and the 22-16 loss at Twickenham in March for the first game between the sides on neutral soil.
He added: "We have every belief in the squad here.
"We will be confident going there. We almost beat them in the Six Nations.
"We're a really confident, tight group and I'm sure when we look back on the game we'll be disappointed, but we can learn from it and kick on."
Barclay repeated something often heard within the Scotland camp last week and something which is sure to be said time and again between now and 2030 on Saturday evening in Auckland.
He added: "It's going to be the biggest game of my career and it should be a great occasion."
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