In reply to Philip:
We have used childcare.co.uk - although more for live in nanny roles. At least you can get an idea of their experience from that, and they should be able to provide CVs etc. Babysitters with no qualifications will be £8-10ish per hour (in London), make that £20+ for qualified nannies.
Failing that you could try putting an ad on Gumtree, but be prepared for some random responses!
Post edited at 14:11