Don’t be nervous, you know/can find the answer to any questions presented. Don’t guess, admit you need to look at regs/viper book to clarify, I still look frequently, you can’t be expected to learn and retain it all. I have a lot saved in iBooks - technical bullitains, unsafe situations etc so they are at my fingertips. You can be knowledgeable but not compitant, compitance comes with with time and diligence.
If it helps.... on my initial meet I was asked a lot about pipe sizing and gas pressure. He looked over my boiler which I fit and asked some basic ‘what if’ questions - all of which were easily answerable.
In my opinion they are basically checking that you are a genuine applicant and not some thicko goon, you’ve passed ACS and demonstrated competence, the rest is just fomality.
Remember he’s not there to catch you out, he ‘SHOULD’ be there to help you along the road to being independently Gas Safe and furthering your career!
Get some decent biscuits in, make him a good brew, be a nice and strike up a repor.
Job done.