
Well done you. How exciting! First of all, don't panic, of course you'll be nervous but nerves can kibosh a good interview - the fact you've been invited to interview means you've met their criteria, and you're a serious contender for the job.
Any good interview will consist of input of 20% interviewer/80% interviewee so make sure you have plenty to say! Ensure you research the job/department well so you know exactly what you're there for. Do not be afraid to make notes and take these in with you (most people go blank when faced with an interview board). You should also prepare a list of questions you want to ask them - an interview is as much for you to decide whether you want the job as it is for them to decide whether you're the person for the job.
Make sure you are aware of any skills that you have which are transferable to this position (i.e. patience/empathy/ability to talk to people at all levels). They will probably ask what skills you have and how they relate to this particular job, why you want the job, why you want to work for the NHS, what do you know about the NHS, they may give you scenarios (i.e. what would you do if.............?), where do you see yourself in X yrs time.
Be confident, be prepared, and most of all just be yourself.
Good luck, please let us know how you get on