Hello Giuseppe! phase 1 at NATS consists in a series of tests. The first test is about the material / documents NATS provides you (25 sheets I think) and a numeracy test. If you pass these tests you take another 2 tests, quote (from NATS): "a spatial reasoning test, and a logical analysis test called Diagramming".
So, you will take 4 tests if lucky
Hope to have helped you
Cheers
EDIT: by the way, I'm also attending phase 1 on Fareham, 15th April at 10:00 AM