Kipfonz : As long as you know what you are doing with your testing i think you will be OK. The questions he asked were about disconnection times, and other stuff that was pretty easy to find in the 17th BS7671. (big red book as i call it)
Just remember to not just blurt out the answers and look in your books first.
One question he did ask was about maximum Zs allowed. And why the numbers in the (big red) and the OSG differ.
It was a few months ago now, but my assessor was more concerned with testing than my book knowledge.
My advice would be make him a cup of tea/coffee, try to find some common ground. Don't kiss up to him, but make sure you come across as a nice guy who is joining the NICEIC only to become a BETTER electrician.
Make sure you have all your paperwork ready for him, and don't stress. My assessor told me he had only ever stopped one guy from working, and it was because he was just so nervous, he was all covered with sweat and his hands were shaking, so the assessor don't want him holding screwdrivers and test leads near the CU.
The assessor is there to help you, not to catch you out.
Good luck, don't stress, and you'll walk it.