I think one thing to bear in mind is that, in my opinion, NATS recruitment is one of the poorest parts of the business.
Not sure whether it's a case of too few people to deal with too many applications, but everytime I was assured that results/interview dates/medical assessment dates would be with me "in 10 -12 days" I ended up phoning them myself after three weeks waiting.
Thankfully once you're in the fold the communication improves a lot.
It is the best job in the world (so far anyway), periods of boredom at the college mixed with long nights revision and horrible practical assessments, potentially with your mates getting the chop. Once at unit it's all worth while.
My advise... keep on at HR for answers!!
Best of luck