The backstory - in 2007 we got planning permission to build a single storey side extension. The local authority nominated an independent building inspector. The build was inspected at the required stages and all that was outstanding was the final electrical certificate. This was obtained but we forgot (stupidly 
) to submit it and being unaware of the potential connotations of not receiving final certification. In 2017 we contacted the building inspector to understand what we needed to do to complete certification. He told us that the file had been archived and there is no possibility of reactivating. Our understanding is that there is no time limit on getting building regs approval though as long as you start building works within three years of the initial application which we did but the building inspector is adamant he can't (or won't) help 
.
What can we do? We're 90% of the way to finalising certification. We know that notifying the local authority would be unwise as they would be obligated to investigate which would rule out achieving indemnity insurance in the future. Is indemnity insurance our only way now? Or is there another option that we haven't considered?
Any advice from mumsnetters appreciated.