If you visit their website, there are details on what's expected of you. This includes keeping up to date records of the electrical tests you have carried out, having the proper test equipment complete with calibration certs, have ALL the books required such as 17th Edition regs, on-site guide, electricity at work regs, erection methods, a copy of schedule `P` (downloaded off their site) and have enough public liability insurance.
The assessors aren't there to trip you up - just guide you through what's required and to make sure you know what you're talking about. I had my re-assessment in June (3rd year in ELECSA), and took my assessor to a recent rewire I'd completed. Just needed to take some sockets and switches off so he could see if back-boxes were earthed properly, that sleeving was in place where required and that rubber grommets were fitted to the cable entries. Then showed him how to do a Ze and PSC test, a couple of questions answered and then back to my place for a cuppa.
If there are a couple of niggles during your assessment, if you remedy them there and then, there's no problem.