It's a certificate to say the install has been done to the relevant standards, complying to building controls.
I assume if you contact FENSA with the cert number they can tell you the name of the installer.
As PBD says, a FENSA certificate only warrants the installer is registered with them & their installation complies (should comply) with Building Regulations; but the small prinmt disclaims any responsibility on them if it doesn’t. It’s not a warranty against either manufacturing or installation defects; it’s a pretty worthless piece of paper really.
The installer’s registration number & their name should appear on the front of the FENSA certificate & it’s they who they should have given a 10 year warranty, preferable via a 3rd party insurance scheme. You need to pursue claims through them if they are still in business; a 3rd party warranty may have some value but you have to pursue the company first & 3rd part schemes usually only pay out if the original installer has been formally wound up; if the company has gone bust, your probably on your own!
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