Like most answers, it depends.
We got stiffed by a guy who worked for a window company. Mate of a mate doing a “private” job so to speak. First question I asked was whether it would come with the fensa cert and I was promised it would.
He replaced front door, rear door and patio door. Front and rear doors weren’t a worry as they were less than 50% glass, but the fensa document never materialised for the patio door despite no end if chasing.
Fast forward ten years when we moved out and the buyer was a real dick, her solicitor even worse and our solicitor seemed spinless, it was a nightmare.
However, there was a fensa document for all the pre-existing windows so I don’t think anyone every counted how many were there and how many were certified.
Besides, there didn’t seem to be anything that her solictor wouldn’t try and cover with an indemnity policy. Didn’t matter if it was already sorted or not. If it had been picked upon, I’ve no doubt there would have been another policy for it. I swear she must have been getting kickbacks on those things.