I had my assement 10 days ago and i have to admit that i was very nervous indeed, more so than my wedding day! (don't tell the wife). But i have to say that the guy from the NICEIC was absolutely brilliant, he made me feel at ease, he told me it wasn't that long ago he was in my shoes. JUST MAKE SURE you have 10mm bonding inside the property to gas and water, 16mm Main earth. Also make sure you print out document P, and all the other paperwork they ask for, they are really fussy about that. Also learn the test procedure inside out, i bought their dvd which was really good, i watched it a 1000 times. Also get green on site guide and green regs.
Also the inspector phoned me to book the assessment himself. I asked him several questions which i think helped my application.
i am still awaiting my registration details so i am very new to this, but i know how you are feeling. Just be confident in your knowledge, he will soon point you in the right direction. He doesn't expect you to know everything just how to find out the right answer in BS7671 17th edition.
Hope that's a help