Great au pair and au pair world are the 2 big sites for applicants wanting to come to the UK. Lots of summer applicants though so look carefully at dates!
When you write your ad be very specific what your requirements are (hours, experience, driving license) and what the job involves. If you want someone to speak Spanish with you or your children you need to make that clear as it will put some people off. If you've thought it all through then it should be a breeze and you'll be able to answer any questions an au pair asks.
For example they might want to know if your start date is flexible, whether they'll get holiday (yes, 5.6 weeks/year but you can dictate when that is), how much they'll be paid (typically £70 for 25 hours work but you want to stay under the limits for tax/NI)), what their accommodation will be, whether they will travel with you, whether they can have friends/family/boyfriend to stay, whether they can have a second job (they can but it does cause extra hassle for you) etc.
If you can find someone to translate your ad into Spanish that should cut down the number of, say, German applicants who apply for every job they see in England as it should hopefully be clear you're targeting Spanish applicants!