We used two agencies: one local and the Norland College agency.
We decided against the Gumtree route as we didn't want to risk leaving our daughter with someone so 'random'.
Our preference was Norland, but we also wanted to give the local one a go to see who they would be able to supply (not least because their fee was far lower).
We wanted to use the Norland Agency as this guarantees a minimum standard of conduct and attitude from the nanny, although of course it totally depends on who the person is, in the end.
Using Norland cost us just shy of £3k, the local one would have been £800, plus VAT.
We found an absolutely superb nanny (via Norland as it happens) and I don't think that we could have spent that money better on anything else.
I am back at work and completely confident about my childcare.
The local agency's approach was to put our job out on Gumtree and I guess that they filtered the respondents. The broader candidate pool would be the same if you did it yourself though and the candidates we interviewed from the local agency were all good quality people.