Why exactly does the work need certifying now? As mentioned, this could be for Building Control, or you are selling and been asked for it, or for some other reason?
I had a customer a couple of years ago who had done a similar amount of work himself, he was also a theatre/stage technician, and was now moving house and had been asked by the buyers solicitor. If I recall, there was an outbuilding supply, with power & lighting, also a new shower circuit in the house. Actually the work was so well done, it was way above the normal standard for domestic work. In this case, I did an EICR for just the relevant circuits (plus earthing/bonding), which was sufficient.