As I said I went for DI and it was quite stratght forward. Firstly make sure you have all the paperwork available for him to look at, insurance, H&S policy, EIC's / MW for jobs, cal sheet for meters, cal prooving sheet, BS7671, EAWR, OGS, qualification proof, etc, etc - they sent me a list of what is needed + OSG, EAWR and some green cert pads after I applied.
Questions he might ask about are safe zones, disconnection times, max Zs, bathroom zones, RCD operation times (300mS/40mS)..... you do not have to know the answers, but you will need to show him where the answers are either in the OSG or BS7671.
Thngs he will probably ask you to show/proove:
Safe isolation procedure
And all other required tests on different circuits - he will ask you to verify Zs results (find relevant table in BS7671-don't try and remember)
He will aslo want to look at a sample of sockets/switches.
Make sure that you have used all the required stickers.
You have to do all the work, he will just observe.
REMEMBER...do not leave your cu open and walk away from it unless it is isolated by an external DP isolator (instant fail). Mine allowed me to as the property was vacant and we were the only ones there, but I still double checked it with him.
Like I said, for me it went very well. No observations at all, quite plesant couple of hours, which came as a surprise.