Advice please on house purchase which doesn't have building control

Hi, I'm after a bit of advice as a FTB. Had an offer accepted in February on a house that ticked several if not all boxes, but the local searches returned last week to unveil that there isn't any building control entries or a completion certificate on the house.

House is semi-detached that was built approx 2008 as a part of 4 or 5 houses (not sure as I think 1 came afterwards). Built by the seller (or they was behind the project).

Our conveyancer has enquired to the sellers solicitor and he said that there isn't a completion certificate but says that the officer from the council had been visiting - but there are no entries. We're not sure what the situation is regarding the other houses and the other house of the semi-detached. 

We can appreciate that the house has been standing for 18 years, and although we have already had a Level 2 survey (which was advised at the time) we'd be happy to get a Level 3 to make sure that it is is structurally safe.

Our solicitor has mentioned that we could either get a retrospective building control certificate or get an indemnity policy. 

In an ideal world, we'd prefer the retrospective building control certificate option as everything would be above board and we'd have no issues worth future projects. But how hard is it to get this? Our solicitor mentioned that if they have plans, photos showing different stages etc it'd be fine all being well but the seller is aging and experiencing some health issues. Is it possible to get it in another way by opening up walls, looking at the foundation, looking at roof structure etc? 

We're leaning against the indemnity route at the moment as we had thought about adding an extension or a loft conversion in the future, and if we had Building Control over to sign it off then that would void the indemnity and we'd be unable to remortgage.

Any advice would be welcome. Thanks in advance