In case it's helpful here is a note of recruiting which I sent to a friend.
"The first thing I would say about recruiting through the au pair world site is that you must be specific about what you want. If you are clear from the outset about duties, hours, housework and how much you will include the au pair in your family then you are more likely to find an au pair who will be happy when she arrives and who will stay and work well with your family.
As a London family you will be inundated with applicants. Decide on a just a few countries you want to recruit from and limit your criteria to those countries. Also decide an age range and ask for applicants only from that range. I personally have had good experiences with Germans, there is a real culture of au pairing as a gap year activity in Germany. Having said that I currently go only for English speaking countries as my youngest is learning to talk.
I look for young au pairs. I choose 18/19 year olds as I would prefer to teach them how I like things rather than have them come with lots of pre-set ideas, plus I find younger au pairs easier to live with. But it's personal choice.
On au pair world I delete all applicants that don't write a bespoke message when they contact me (some use generic messages generated by the site). If their message doesn't mention my kids I delete them, I want them to show they have really connected with what I have written in our profile. This immediately leaves me with a long list. I then email a few basic questions:
Why do you want to be an au pair?
Why London?
Tell me more about your childcare experience.
What kind of relationship do you want with your host family?
If I like their answers, and they ask good questions in return, then we arrange to Skype. On Skype the key is deciding whether you have a rapport. Au pairs become part of your family, you spend a lot of time with them, so rapport is important.
Think carefully what kind of person you want. For example, do you hope they will go out a lot and give you space? If so look for someone who is obviously outgoing and sociable. But consider whether returning from late night parties will bother you. On Skype I ask the following:
Why did you choose au pairing over other gap year activities?
What kind of activities would you do with the kids on a rainy day?
How would you deal with the kids fighting with each other?
What's your approach to discipline?
Can you cook?
What do you like to cook?
Is there anything you don't eat?
I ask for light housework. Are you comfortable with this?
Are you a naturally tidy person?
What were your own hobbies in school?
What do you hope to do after au pairing?
How much time do you want to spend with us as a family?
What do you hope for from your year in London?
How will you spend you spare time?
How will you make friends in London?
Have you lived away from home before? Did you get homesick?
If I like their answers I bring the kids in and let them Skype and watch how they interact with the kids. Then I make an offer which includes a clearly written set of hours and tasks plus pay and holiday details. Having au pairs has been amazing for us. Our kids love them and I find the flexibility of live-in help is a god send. I hope that is helpful. Good luck with your search!