You would need to go back to the company that installed the Windows and ask them for the certificate and they will probably have no records so can’t do it.
The next course of action is building regulations approval although it’s out of time for enforcement now as it was 17 years ago.
The easiest way of dealing with this is just to provide building regulations indemnity to the buyer. Not expensive, and quite normal and acceptable to 99.9% of solicitors although you will get the odd one that awkward because he “doesn’t like indemnity policies”.
In the latter case, the only alternative is to find another buyer or apply for building regulations approval.
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