I'm also struggling to find an au pair. I keep on having them flake out/not turn up to interviews. Of those that I have to interviewed, and offered the job to, I have had them all reject us.
We live in central London, and no driving or weekend work is required. I'm willing to accept any nationality or age, as long as they can legally work in England. I'm really not being fussy!
I have 3 children, so I'm not sure if that is what puts people off. We are offering what I believe is a generous package, but am curious as to what others offer.