As said nothing to do with invoices,
I show all my certs for previous year, he will only want to see domestic certs/jobs if you are going for DI,
see the check list they send you and make sure you have all of it in a nice folder so its easy to look at.
you will need at least 1 notifiable job, preferably a rewire of at least a cu change, and 1 other, could be a minor works.
safe isolation will be expected.
ze & pfc
you will need to know the limits for the tests you will be asked to perform, you will not be asked to do insulation res test but will be asked how to perform it and what results are acceptable and what is not.
you will be asked for max zs (80%) for either a 6a type b or a 32a type b or both.
you may have to test a ring.
you will have to test an rcd and explain the readings you get and whats not acceptable.
you may also be asked to explain entries on the certs you show him so make sure they are all ok or you could be picked up on the errors.
you could also be asked anything from bs7671.
will take half a day but i never book any work in for that day as i don't want any distractions.
hope that helps