Berni, they never had A levels in your day. Higher is a word that no-one ever associated with you.
Craig, consider what you will do if you don't get in to ATC. Best and safest bet is to apply for ATC, but also either go back to school to improve grades whilst application is processing [you can always jack school early if NATS takes you or postpone joining NATS until after you resit the Highers next May] or if you are offered a place, start at Uni. That way if NATS gives you a GTF, you can still get better grades and/or nip off to Uni instead. The grades you have must give you a lousy choice of Uni places at the moment.
The grades in your Highers don't count for anything with NATS. You have as much chance of getting in now as you would with 5 A's.