Well just remember that he is there to PASS YOU , not to FAIL you as your going to give him a lot of your hard earned so relax a little.
I always found that I can tell if a guy knows what he is talking about electrically and his ability within the first 5 minutes, he will be the same because that is his job, and again remember he is there to PASS YOU.
Depending on what he discovers in that 5 minutes will dictate your assessment. If he feels your weak on something he will press you on it, and then most likely give you advice and point out where you are slipping, but remember he is there to PASS YOU.
As it the NICEIC they have strange concepts of things. So overplay the safe isolation they hate people working "LIVE" even if "LIVE" testing is not "LIVE" working. So gen up on that and the dead tests. Remember he is there to PASS YOU
Also have all your docs there, they asked for, they love a doc so the Part P approved Doc, your calibration cert, the EAWR-1998, Regs and if you have them a few of their over priced "Snag" books, and remember he is there to PASS YOU
If you don't know something don't waffle it, tell him your not sure and look it up. so get to know the regs, OSG, Electricians guide to the building Regs, and remember he is there to PASS YOU
Finally I'm sure there will be some testing he will ask you to do, most likely a dead test as they don't like the "LIVE" ones, oh I covered that, Have I mentioned yet that they are there to PASS YOU!!!
So try and relax, if you know your stuff, he will know you know it, and it will be a breeze, just 2 old sparks having a natter