Cavity narrower than expected

[overstressed]

Hello,
We are having an extension built. The wall is made up of an inner block leaf, an insulation layer (60mm ecotherm), a cavity, and an outer block leaf, with 25mm of external render on top.
The cavity was supposed to be 50mm wide but the builder made it narrower (approx 25mm). He insists that it's not a cause for concern, as there won't be any water penetration from outside and the condensation buildup in the cavity is minimal and no issue for inside.
Any thoughts?

Don't listen to your builder entirely, talk to your architect too. You architect has a professional responsibility to make sure it's OK, the builder just wants to get on with the job!
And for what it's worth our architect said the cavity is extremely important and the water has to drain to the outside to avoid damp coming through in future.

Yes, of course. The architect (who was the one that actually spotted it) says it's not built according to the standards, therefore he would have it rebuilt.

This could very well be the best course of action, however it does have costs and before I do that I want to weigh the certainty of this extra cost and inconvenience in the short term (extra rent for the additional waiting time and emotional cost from not making it to our new flat this side of Christmas) against the probability of issues at some point in the future.

What is that probability and what could happen and at what cost? If for argument's sake the current cost from the delay (not including emotional stress) is say £2k, and there's 50% chance that an issue will arise in 10 years that it will cost me £2k to fix, I'd rather take the chance.

So, the question is what can happen in the future?
Is the damp going to come from moisture through condensation alone (in which case it's quite likely to happen) or will it require direct water penetration as well (which is less likely if not impossible due to the thick block+render, assuming good maintenance of the render/cracks repair etc)?
Next consideration: let's say water forms in the cavity, whatever the source. How is it going to affect me? Behind the cavity gap, there's the insulation block, behind which is a damp proof membrane, and behind it is the inner block. The only direct connection is through the wall ties. These are stainless steel and stop at about half the depth of the block. Is the concern that the water will moist the inner block and then it will travel inside?
If that's the case, then can condensation alone, traveling through the wall ties to get to the inner block cause any problems (especially when presumably at that point it will meet higher temperature from the heated interior, which will dry it)?

It's a tricky situation. The "correct" thing to do is to rebuild the wall. But before I do that I would like to be presented with a good explanation of what can go wrong (at least one scenario) and why, and weigh the odds. Otherwise it's choosing to be pedantic over being practical, and it's sort of cutting the nose to spite the face.

If the builder made a mistake and built it incorrectly, not in accordance with the plans, shouldn't he be the one to shoulder the burden of the extra work?

I wouldn't pay him more money because he made a mistake.

Yes, I'm not talking about the rebuilding cost, but of the cost of delaying our move-in date (at the moment we are renting elsewhere and waiting for our build to finish to move in)

I think a 25mm residual cavity with 60mm of ecotherm is ok, actually.

If the residual cavity is clear of snots and debris and the wall ties have been fitted correctly, I can't see an issue.

In your shoes, I would be worried of gettig sued. If you sell, a problem arises and the buyer find out you sold knowingly hid the issue you might find yourself in hot water.