What is the point of an indemnity? it's hogwash. It indemnifies only against action by the LA for unauthorised works that should have been notified. After 8 years, not going to happen. It's not an insurance against bad workmanship and has no mechanism to pay out for further building work to repair. The buyer should survey the house and value it accordingly. Personally, I would say nowt, do nowt - if anyone asks when the work was done, say you have no idea. After all, you say you bought it like that, so could have been done years ago (pre 84) before BC notification was required.
We sold our last house 3 years ago. It had had major surgery some time in the past (prob early 90's) - no certs. Buyer asked, we said no idea, Checked with LA but no records, buyer still bought.
You've lived in it 8 years. If it was going to move there would be some signs by now - cracks etc. If it's all solid after minimum 8 years, it's most likely absolutely find and was done properly cert or no cert.
The reality is that most LA's won't have a clue whether the work was certified or not if the records pre-date their digital system, irrespective of whether technically they should be able to tell you. If it could be in paper archives, it might as well not exist.