Looking smart without being a smartass is the key, I think.

You should be open and confident. You should keep in mind that you've got energies to get a job well done: if it's this one, then great. If they choose somebody else, that doesn't mean anything but that you'll start working somewhere else.

Success!

Most important thing is to throw away all nerves at the door and say "let's do this" sych yourself up.

Body language is very important, probably more so that what you actually say.

As awkward as it can be try to use as much as eye contact as you feel you can get awy with without feeling weird.
Don't sit on the edge of the chair, sway back and fourth (lol obviously) slouch. Have a good posture.
Use your hands when describing something, be a bit animated.
It might just be a supermarket, but dress to impress!!

erm what else ..

When they ask "any questions" try and have at least one in mind, so think of something before hand, but PLEASE,  not "how do you handle sick pay?" that's just ... no.

Edit - Oh and if the interviewer asks "what's your weakness" make sure you prepare an answer that pans out into something that's positive, i.e "Oh I think I put too much effort in, I work too hard" ect

Doesn't have to be original, the question isn't original and redundant, I feel when I get asked that in interviews they're inept.

Edit 2 - I realised you're in the UK, which more than likely means this'll be a group interview with more than 6 other people in the room, job centre or not.

Hopefully you're on your own or in a pair at the least, because they arnt fun. You need to prepare yourself for this and not have it thrown at you, that can really throw you and how you are in the interview, if you expect it to be 1 on 1 but find you need to talk in front of 5 or more other people!

In regards to group interviews, they go for the bubbliest and loudest from what I know (only been to one, never again.) so if you're determined to get the job, let loose and pretend they aint there.

wear a tux

here, ive found you a good instructional video on job interviews

so how did it go,

i remember having an interview at the job centre for a dept store,but thankfully it wasn't a group one

yes,they had over 400 people aspply to the interview i went to aswell,alot of people for the jobs advertised

Act gay