For an interview, its shirt tie and suit always. And there are suits and there are suits. Dont wear a shit looking one and dont wear it like a twat. The following rules are important:
get a normal fit work suit, not a skinny fit one and definitely not one of those god awful shiny ones. Dark blue is good, the brighter blue ones Ive seen about recently are not. Do not spoil it by being one of those flash cockknockers that wear the same colour shirt and tie. Not wishing to sound unpleasant, but if you dont dress smartly often it is easy to look like you're going to your court hearing. As for ties, avoid skinny ones, avoid massive knots, and get a shirt with a collar you do up comfortably and do up the top button ffs. I see lads who turn up for work with me with ties hanging down and top button open and think there is no point, its one or the other. Dark blue suit, white shirt, tie, clean shoes. The job will be half yours just for making the effort, trust me. Its not as old fashioned as it sounds, its the small things that count when you are trying to be the best candidate in a group of applicants.