Hi Bubbah.
If you go back and look for the NATS interviews thread you will find loads of good advice. I got through (although I'm still not sure how) and all the advice I would give is
a) Make sure you've had loads of sleep
b) If you really want the job, make sure it shows
c) Know the basics well but don't get paranoid - as some wise ATCOs told me it's called an interview not a test, and if you knew it all already you'd be doing the job
d) Relax and think before you act/speak - if you're wound up you'll blow your chance for the year!
Good luck and just believe that if it's meant to be it will happen.
Let us know how it goes and maybe see you in July,
G-BJOP