Childcare.co.uk and findababysitter websites were v useful for us (the 'babysitter' site has heaps of nannies / childminders registered, some with reviews from previous families). As well as placing your own ad on these sites it's worth searching through the ads the nannies themselves have placed. The sites seems to be well used so keep searching as new listings crop up pretty frequently.
Gumtree was a bit of a mixed bag, with some good responses but a lot of people with v little experience so best to only list there if you've time to wade through the chaff!
We also tried agencies and I was so surprised at how poor their response was, so def a last resort.
What hasn't been mentioned so far is putting small ads up at local playgroups / cafes, this came up with some surprisingly good candidates; also asking around - put the word out amongst friends / friends of friends.
Use the nanny network - when you speak to a potential nanny, if things don't work out with them ask them if they know anyone else who is available.
Also, ask your local family (children's?) information service. They provide a list of childminders with vacancies but also provide details of Ofsted registered nannies with vacancies.
Best of luck with your search