Don't worry they are usually quite friendly and ultimately they want your money!
Be organised, have ready for them:
-public and professional liability insurance documents
-calibration certificate for your tester
-18th edition certificate, and any other certificates for training you have received
-I think they wanted to see that I had a regs book.
-you need a job to take them to (which can be a CU in your own house) and an EIC for it. They will ask you to do a couple of tests, (often a Zs@DB in my experience). I think it's a minimum of 2 new circuits or a CU change.
They will probably want you to watch a promo video for their Fast Test software. Just politely sit through it!
(On a different note, I honestly would consider dropping 3rd party certification next year and save yourself some money. I had it for my first year as they sold it well on the phone, but in reality its such a pain to use that I didn't once use it.)
All the best with it, and tell us how it goes!