The first point to note about a missing NHBC certificate is that if the certificate relates to a new build property which is more than 10 years old the guarantee would be out of date and may be unnecessary unless of course the NHBC also signed off the Building Regulations approval for the building works.
Where the NHBC was covering Building Regulations you will need to keep a copy of the certificate even if the building was completed more than 10 years ago.
In the event that you have lost your insurance certificate then the NHBC will issue a replacement for a small fee. Sometimes this can take a couple of days to come through but it quicker it may well be to contact your previous conveyancing lawyer or property solicitor and see if they have a copy on file.
Even if you are only able to obtain a copy from your previous conveyancing lawyer then the likelihood is that the buyer’s property solicitors will accept a copy on the basis that the original is handed over prior to completion.
This means that the absence of the original certificate should not hold up exchange of contracts and in the meantime you can obtain a copy of the original from NHBC.