Where exactly does this 'factory mis-build' show up? Is it somewhere where a potential buyer is likely to see it, or is it just on a Renault computer somewhere?
The part that was not built properly, is a brack leading down from a tubular bar across the dashboard, going down to what you might call the tranmission tunnel. hidden away behind plastic etc, no way a buyer could have seen it.
As long as it has been repaired to the proper standard, I see no obligation to declare this to a future owner. As a courtesy, you might mention in passing that it had a fault from the factory that was rectified by the dealer. I don't think you'd need to disclose any more than that.
This work will remain on record against the car within the Renault dealer network. Not as easy as that to hide.
So if the car ends up being repaired to your satisfaction and the resale value remains unaffected, I really don't see why there's any cause for concern.
I'm not satisfied that it is a repair, rather it is a modification. Resale value remains affected by the above about it being available information through renaults computer system.
Mord may well be right that you could reject the car if you wanted to, though you may find yourself liable for a deduction for the amount of time you've had it and mileage that you've put on it. Is it really worth being out of pocket just as a matter of principle? If you're otherwise happy with the car, I'd recommend that you let them fix it, and then put all of this behind you.
The amount of time I've had the car does not detract from the fact this is an issue since the day the car was built, and as per the wording in the Supply of Goods (implied terms) act, "would not have been apparent on a reasonable examination". (Section 10, Subsection 2C, point C)