So This is my second time applying for NATS, last time I failed on the competency based interview, I have my stage 2 coming up on the 13th of Jan, hopefully I'll make it back to stage 3 and pass where I went wrong last time.
for those doing stage 2, theres not much you can do to prepare for it, what I did last time for prep was find games that involve you multitasking and ask a friend to read out simple maths questions to you and ask you to wait 20 seconds to give the answer while still playing the game on your iPad or what ever. Felt this helped the most. As for Stage 3, it's tricky. I can't give any advice as I didn't pass last time, just be prepared for tricky questions, and what ever you do don't panic (Like I did).
Good Luck all