If you are going to be late, ring your AFCO before you are (afterwards they kind of suss it)
Dress smartly- first impressions count.
Make sure you take all documentation requested (Record of Achievement, certificates, referees detail if you haven't already provided them, 4X passport photos, passport, birth certificate, driving licence, NI Card, NHS Number card, etc).
Be yourself, be 100% truthful, try to relax, but avoid leaning on or putting your feet up on the interviewers' desk!
If you don't know, say "I don't know, but I'll find out". Do find it out. Don't waffle.
Good luck- you'll wonder what you were worried about afterwards!