I'm really sorry but I can't give you good news.
Come what may, you signed this off. If you did sign this off then they are not responsible.
You could try to argue that the CADs didn’t match the quote and that amounts to negligence but the problem remains that you were shown them and you signed it off. There is just no way around that.
If they hadn’t been manufactured then probably you could dig your heels in to put pressure on them but if the windows have been made already they will not almost certainly.
No you may not.
But you signed this off.
I don't know whether it can be amended? If so, you could try to argue with them over that.
I wouldn't take any notice of the CAB. I have just been dealing with somebody who was given completely incorrect advice by them.
In this instance, they are grasping at straws.
You signed this off. Even if they were wildly negligent, you viewed it and signed it off.
That is why they get people to sign things off.
It is a little unfair because nobody really studies these things but the reality is that the law will presume you have done so if you have signed it.
I would still try to negotiate with them.
They might well do more than the law would require.
Well, if you haven't paid yet then it is in his interests to negotiate. Otherwise he will have to sue for all he knows and that carries risks.