Thanks for all the suggestions guys.
I have a contact within the organisation who will be present in the interview, (she's mostly the reason I landed the interview in the first place, I imagine). I just asked her to confirm what the situation is, as I didn't want to insult the company ethos etc.
She confirmed, smart jeans, smart shoes, a smart shirt, absolutely no tie, and no suit jacket. Think I'm pretty much set! I didn't want to get in touch with her really as I didn't want any "outside help", or any more than I've already had, but she was more than happy to advise.
Just need to wait for the interview to roll round now
I'll let you know how I fare.