Hello mate. May or may not apply but I placed a couple of guys at an IT firm the other week. They were technical advisors.
The assessment day took the form of a group discussion where you may be marked on your problem solving, personality and contribution.
They were then set a 1hr written assessment, relevant to the job role. During which, they were pulled out twice to have a more formal interview with either the assessors/ hiring managers.
Like I say, it may vary, but this is the usual make up of an assessment day.
Don't worry about it, just be yourself and I'm sure you will sail through.
All the best