All candidates will be nervous, and the interviewers will be very used to that. They'll do their best to make you feel as comfortable as possible. It's in their interests to get the best out of you on the day.
Some random advice:
• Let them know that you're particularly nervous at the start of the interview. (For example, "I'm very nervous, so apologies in advance if I stumble over my words.")
• It's not a race, and there are no prizes for answering more quickly than another candidate. So take your time.
• Before each answer take a breath and pause for a second or two. This will give you a moment to think, and should help calm your nerves.
• If you find yourself stumbling over you words, then stop talking for a second. Take a breath, and then continue what you were saying.
Reading the above back, I realise that it's not ground-breaking advice, but hopefully it's better then nothing!
