The bottom line is they want your membership money for many years to come, and it's exceedingly rare to completely fail an assessment to the point they don't let you in. Worst case for most is another assessment within the first year.
Best advice I can give is relax, be honest, and look organised.
Have things out and ready. MFT, calibration certificate for it, public liability insurance details readily available, certificates for any qualifications you have told them you have readily available, print out their daft blank customer complaint form, have the EIC for the job you are looking at out and ready.
They might ask you to demonstrate a Zdb, or an rn.
If in any doubt about the assessor being a over-zealous wally then turn off (with householders permission) and demonstrate safe isolation, remembering to prove tester afterwards.
(My last one was a very decent bloke, I said "ok to leave it on?", he nodded!)
All the best for it, I'm sure you'll be fine.
You are the first I've heard of managing to join DI under new rules btw, when it's all done and dusted we'd be interested to hear of what the process entailed. From anecdotal rumours it's a miracle they were organised enough to phone you back!