When I was working full-time we went through APNI (
http://www.apni.co.uk/). They provided a really thorough file on each au pair, with photos of them in their home environment, details of experiences and many many questions that I'd never have thought of. We emailed potential au pairs and then skyped them to interview with our daughter.
We had a French one first of all who we interviewed in French (we are a French/English family), she seemed fine but after about 8 months she had really gone downhill and just didn't care about our daughter (various things happened which I won't go into here), so we let her go. We went back to the same agency who were totally on our side and helped us find a new au pair, even reducing our fees as it hadn't worked out with the first one.
We then had a German au pair who was AMAZING! She stayed for 15 months, and 2 years on she's been back twice for holidays as we all got on so well.
Both times the au pairs lived abroad and then moved straight in with us when they arrived in the UK. Every time when hiring and firing I've gone with my gut instinct above anything else.
Good luck with it x