This is not something I have come across before, but you should not have to accept this.
Just because a manager that agreed replacement was justified has left the company doesn't not mean they can then refuse. Do you have anything in writing regarding he replacement?
Most accept the ruling of the GGF, however it is funded by the glazing industry and its independence is easily questioned. I would also add theta the GGF state that and scratches must be within a certain area of the window be visible from certain distance away not in sunlight, which bats away most glazing complaints.
I am more impressed with the original explanation as it sounds plausible to me. You could always get your own glazing expert to inspect and write a report, using this for court action if it becomes necessary.
Finally, I find it very suspicious that the are only giving you the odd paragraph from the GGF report even thought hey stated you would have a full copy. Obviously it might be that many others also have the same issue as you and they don't want you getting in touch and having strength in numbers. Siting Data Protection is no excuse as they can easily redact the names and addresses which would be a breach leaving you with the meaty parts of the report which they promised.
I smell a rat. Time to get the pest controllers in! Get a solicitor and an independent expert.