I would say don't get too stressed about it as long as you've done a good job of the paperwork and the actual installations then all will be well ,if not then thats a different story
I had my first visit for approved contractor (been a dreaded DI for a few years) last year, no preassessment they phoned me up on a Tuesday evening a couple of weeks before the due date asking whether I would be interested in having a visit the following morning as they had an Engineer at a loose end for the morning ,after a cancellation . So I thought about it for an hour and rang back saying yes I'd do it as long as I wouldn't be charged if a revisit was needed, they agreed .
So I just spent a couple of hours when I got home making sure the certs, text books and other documents etc were all in order, made sure I had batteries for the test gear and rang 10 customers to arrange a potential visit as I thought he'd pick them at random filled out a sheet with their contact details so I could ring them with no fuss, just doing these few things seemed to impress him enough (might have been the chocolate hobnobs always a good idea) that I was prepared for him, answered his questions and asked him a few, even disagreed on a couple of minor things no big deal at all as long as your competent and prepared you'll have no problems ,it was easier than I thought it would be.