I haven't worked for a council, but having been to a few interviews in the past I can say that they tend to be small, maybe only one or two people with you. For me I find that a lot easier, as it's far quicker to break that social barrier that stands between you. Obviously you don't want to take it too far, but just remember that they are people as well and turn the interview in to more of a chat than a questions and answers session.
Next, make sure you know as much about the position as you can before you go in. You want to show to them that you are interested in the position enough to have done your homework first.
Also, go in with a few questions. It's just as much your time to ask things as it is theirs. And again, it shows that you're interested in the position.
As for nerves...well, there's not a lot to be done about them. I would suggest taking some herbal calmers an hour or so before the interview. I say herbal because the ones that I have used in the past have been great without making me drowsy.
Oh, and that reminds me: Have a good night's sleep the night before! It may be difficult to do with the excitement of the job interview the next day, but it always helps to be rested rather than sat in the interview yawning because of the heavy drinking session you had the night before. But I'm sure you already guessed that one.