I was in the same position as you last year, so have learnt a lot in a year!
I used agencies last year when I got an au pair for the first time, and again this year when I got my new one - they have given me much more confidence that I have found someone I can trust living with me and helping to look after my girls. The agencies have relationships with au pair agencies in the other countries, so all applicants are screened locally, and depending on the agencies, are interviewed, vetted with the local police, health-checked, with childcare references taken up etc. You will still need to interview the aupair over the phone but its much safer to go through an agency (at least the first time so you can see how it all works)
My advice is to register with more than one agency so you get access to a wider range of available au pairs - especially if you need someone to start by a certain date. There seem to be plenty of girls about but most didn't want to start until end Sept, which was no good for us.
The agencies I used this year were angel au pairs (tel 01252 851978) and millennium au pairs www.millenniumaupairs.co.uk - though i think they may possibly only operate in the South East. I found their details from my local Family Grapevine directory - here's the link to their website www.thefamilygrapevine.co.uk/publications.htm, it's not particularly user-friendly but it's worth perservering to find your local childcare agencies)
Once you've found an au pair it still takes a couple of weeks before they arrive, and then a few weeks for them to settle in and get to know your children, your house, your routines etc - and very importantly for you to assess their capabilities and learn to trust them with your children.
So my advice is start looking asap, then there's one less thing to worry about as you prepare to return to work.
Good luck!