pugwash,
I'm sure some doubt comes into everyone's mind at some point, especially when the pass rate is mentioned - even if you get to college, will you make it back out still as a NATS employee?!
Visits are relatively easy to organise as long as you're willing to be flexible and travel a bit. I'd suggest a visit to both an aerodrome site and either Prestwick/Swanwick (and there are of course other non-NATS aerodrome locations too...). If you make it through the selection process, you'll be asked whether you would *prefer* Aero or Area (no guarantees!) - so it's best to see what they both do.
Either ask around on here (or in the interview thread) or contact HR who can put you in touch with the right people.