We have had about 8 au-pairs in London. They key thing is to find someone who fits well into your family as generally they do eat with you and you end up spending quite a bit of time with them in the evenings.
Do Skype interviews, and if you have the slightest doubt, don't do it. We have had one disaster which was traumatic for everyone.
As they live with you, are young, possibly away for the first time, it is vital you are willing to let them become part of your family. To successfully achieve this, they must be compatible and willing to adjust to your way of doing things.
Also, it is essential they get their own social life so they aren't totally dependent on you. There are facebook groups called 'au-pair in London', that have regular meet ups. Get any au-pair onto these before they arrive.
For the first couple of weeks, we've found au-pairs do spend more time with us in the evenings and weekends. By the end of the first month, the dishes are done and they're in their room surfing / watching telly / doing homework, or they've gone out. Weekends we rarely see our au-pairs - they're partying, sleeping, hanging out etc... They sometimes eat with us, but not often. Unless we're doing something really exciting they don't want to come - they'd rather be in Camden with their mates!
Over the years, I've learnt what is most important is getting the right 'fit' for the whole family - not just the kids. This is make or break. Having someone around who has a totally different idea about life, expectations, ambitions and so on takes a lot of effort as no assumptions can be made. It's harder for them and you.