When I had mine, I just showed him about 3 boiler installs and he made me run through the commisioning procedure on one of the jobs and made sure all readings were correct. Also got asked to do a tightness test. Other than that he was just generally talking about gas safety and asking me questions, discussing situations and what I would do if said situation occured. All pretty basic really, he was a nice chap aswell. It didnt seem like he was trying to catch me out on anything. If I was unsure of an answer I just said I'm not 100% I would have to look it up in my book and showed him I knew how to find the relevant info and he was fine. You arent expected to know everything off the top of your head, thats why we all carry books with us on the van.