Hi Annie,
I am a nanny myself and I would advise you to get as much info online as possible, try this link
http://www.babycenter.com/0_nanny-inter ... 1450905.bc for some basic pointers.
I would also brainstorm, make a list of whats most important to you and your family as there are some great nannies out there but still might not be the right fit for your familys needs.
So apart from previous experience, CRB and first aid training, I would want to get a feel for the nanny as a person and foind out what her approach is to childcare.
possible questions
1. why did you choose childcare/nannying as a career?
2. what is your approach to discipline?
3. can you give examples of any highlights or happy moments in your previous nannying jobs
Customise the questions so they fit for your family and children, think about what is most important to you ie routines, if nanny is comfortable to work overtime, do light cooking etc.
Don't be afraid of being thorough, the nanny will want to know as much about the role as you will want to know about her ability to be the perfect nanny
good luck