Hi mean,
Congrats on the invite to assessment day. The technical test on my day didn't involve programming, although i don't really know if the questions keep changing in these tests. They were just general questions that don't require much prep. They are no clear cut Q+A types. I suggest you go through the website about what they do and their overview and basics to help you with it. I had hard time with the questions because i hate writing and the time was kind of not enough to think and write.
The people at NATS will make you instantly comfortable, so you don't have to worry about nervousness and all that stuff. Be confident, interact well. Try and get to know your fellow applicants 'coz it really helps you guys to click in group exercises
Anyways, best of luck. Do your homework, attend the assessment and hopefully, share your experience with me .
Cheers