Brian Smith: The Welsh goat is just annoying. I once hit it with a ball but it ignored me...

Rugby orthodoxy tells us that the biggest games, particularly games on the unusually grand scale of this evening’s Six Nations decider between Wales and England in Cardiff, are won up front. But orthodoxy is not infallible.

The way I see it, the crucial contest at the Millennium Stadium will unfold a long way from the scrums and line-outs, with the respective back-three units – Leigh Halfpenny, Alex Cuthbert and George North for the home side; Alex Goode, Chris Ashton and Mike Brown for the visitors – being the key figures.

This is not great news for England, who will be fielding two full-backs and an out-of-sorts try-scorer against rivals who have been looking more threatening by the week. Speaking as someone who has performed the attack coach’s role at international level, I have to say that I love the way Wales use their wings, particularly their blind-side wing on the “second runner” play. From the defensive point of view, one wrong “read” can easily cost you seven points. There is very little margin for error.

Look at it from the perspective of Brad Barritt or Manu Tuilagi, the England centres. Wales have big, powerful midfielders; certainly, Jamie Roberts is bigger and more powerful than most. When Roberts comes stampeding into your channel, you have to stay strong on him because he has the size and physicality to run right over you. But more often than not, he also has someone like North, equally powerful and a whole lot quicker, coming out the back. The longer you stick with Roberts, the less of a shot you get at North. And believe me, a last-ditch arm tackle on a wing of his proportions is destined to fail.

There will be fascinating individual and unit battles all over the park, but to my mind this one is the real concern for England. Both sides are super-efficient on the back foot – Shaun Edwards and Andy Farrell, former rugby league men who have so much in common, pride themselves on building systems that produce clean exits from defensive situations. It therefore follows that this could be a low-scoring game on the try front and, as the statistics tell us that two tries in a match of this magnitude will probably be enough to secure victory, you have to ask yourself which of the teams is best equipped to score them.

For my money, Wales are more dangerous in the final third of the field with the advantage when it comes to ammunition. If Ashton, the only natural wing in the England side, was in really good shape, the balance might be more delicate. Unfortunately, he’s going through a lean spell. When he’s scoring heavily, it’s possible to make allowances for the things he doesn’t do – to forget his frailties, if you like. When he’s not scoring, the fact that he is not a “complete” wing is harder to ignore.

What will be equally hard to ignore, in my experience of international rugby on the far side of the bridge, is the fun and games before kick-off. On these occasions, a Welsh marching band, invariably featuring a goat in ceremonial attire, appears on the pitch just as the pre-match warm-up reaches the critical juncture. And guess what? The musicians and accompanying livestock always materialise in the half of the field occupied by the opposition, rather than the home side. Coincidence? Please.

The parade goes through the in-goal area before heading diagonally towards the halfway line. No matter what preparations you may be engaged in, they’re coming through. The result is that you give it up as a bad job and head back to the dressing room a good 10 minutes earlier than you intended and lose every last drop of competitive juice flowing through the team. There isn’t a coach in the world who wouldn’t be upset when something like this happens, because we all have our little routines and we don’t like them being interrupted.

I can remember kicking a ball at the band leader, but it had no effect. He just smiled and kept marching. One time, I even managed to hit the goat – a particularly good shot, that one – but it ignored me and walked on. When we won down there two years ago with one of the best performances of my time in the national set-up, I went looking for the goat afterwards with the intention of giving it a slap on the arse, but then I remembered the size of its horns and thought better of it.

That game in 2011 was the first match of the tournament, not the last, but it still generated a hell of a lot of heat. It goes without saying that this one will be hotter still, given the prizes at stake. If England can bring it down to a kicking game – a penalty shoot-out – or find a way of making it a one-try contest, they have the character to come through what is certain to be a major test of their resolve. But if Wales score that second try while the contest is still in the balance, I fear for them. The firepower among those red-shirted outside backs is pretty formidable, and for all the intensity and organisation England bring to their defensive work, they may find it impossible to resist.

Brian Smith is rugby director at London Irish and former England attack coach. His fee for this article has been donated to Great Ormond Street Hospital