aidob,
Having interviewed NATS Direct Entrant Graduates and Industrial Placement students for a number of recent years, I'd say that there is some pretty good advice been posted here and if you follow it you won't go far wrong.
I'm not interviewing EngTechs and I haven't a clue what they will ask about, but if the radar question comes up, know what Mode S is when it comes to Secondary radar and you can't go far wrong.
Good luck!
RS