Super Duper Fly - with regards to how they fill the day, I'll give you a bit of an insight - well how it was for me at least!
On my stage 3 back in November, there were 6 of us in Manchester. I think it commenced at 8.30am, and to begin with, we had the introductions from the NATS staff present (2 ATCO's and 2 HR staff), and then the six of us did the usual introductions you've probably done a million times before at interviews, induction days etc.
From here, the group exercise was explained to us, and then we commenced with the exercise which lasted 30 minutes, and was followed by a 5 minute group chat about our performance - what he had/hadn't done well. Throughout the whole time, the assessors were pretty quiet, busy making notes on whoever it was they had assigned themselves to.
We were then informed of who would be interviewing who (an ATCO and HR member per interviewee), and that between our group of three, we should decide in which order we should go. Two per group were to be done before lunch, and the last member of each group afterwards. In this time, we were free to do whatever we wished - leave the hotel if we pleased, or stay like we did (I, along with another girl, volunteered to go last, as we were both fairly local and didn't have to travel far unlike some of the other candidates).
If I remember correctly, lunch was around 1pm. Over lunch, we had the opportunity to ask questions on things such as what would happen after the interviews, how soon we would start if successful, wage structures, basically all the things that you read on here as rumours! Once this was over and done with, the four that had been interviewed were free to go, leaving the two of us to have our respective interviews.
My interview lasted about 1hr 15mins, which was pretty consistent with the other candidates, but it really flew by, as there always seemed to be someone talking. In fact, I actually found it quite interesting and stimulating, just for my general ATC knowledge. I was finished and on my way home by about 3pm, but if you're lucky, it could be as early as 1.30pm.
I'd just add that, personally, after my interview I felt really demoralised and thought I stood no chance of having passed. I got a real grilling and looking back wished I'd given different answers etc, but having been successful, I must have said something right. So on that, just give it your best and if you don't feel great afterwards, it doesn't necessarily mean bad news is coming your way. Just be confident in what you are saying, as the interviewers may throw in certain "hints" that may prove to be red herrings, just to see your reaction. Unless you can see a certain sense of logic behind what is being suggested, then I'd say your first reaction could be the correct one, especially if you can justify it.
Hope this is of some use, sorry to have waffled on a bit! If I can help in any other way, PM me, as I'll be more than willing to help, as are most of my new colleagues!!
GOOD LUCK!