Sorry, I've just looked, and this is a bit of an epic.
Like someone just said - phone interview. I put together a list of questions to go into things like basic common sense, etc. It gives you a chance to weed out anyone who sounds like a real muppet - you'd be surprised what people will say on the phone. You can also schedule it in the evenings.
I tend to do a couple of sieving 'tantrum', 'choking', type questions in there - along with eg, what they think their strong/weak points are and what makes a good nanny. ie, half competence/child understanding and half psychology. I run it a bit on feel and following responses, and a bit on a pre-determined list of questions.
And it gives you a chance to make sure the timing of the proper interview is good for you both. For me a candidate saying 'can't do the interview - have promised to do agency work for X nursery that day' means they wouldn't let you down. It also gives people a better chance to understand you as a person, and what the job involves.
My only experience is with Gumtree/nannyjob diy recruiting. It seems to take a bit longer I think.
You do get strong candidates through Gumtree as well as real muppets. (My current nanny just came through Gumtree - though it's still early days) For me, all the last 2 years has to be in childcare/college, unless there is a clear reason for picking up a short term job. And I like to see at least one sign of responsibility on the CV too, if they have no sole charge nanny experience. (eg, room leader, preschool teacher, running a youth group - something that says 'organisational ability in some form', or some volunteer work. Something that says 'I'm more than just playing with children under constant supervision').
Another thing I've filtered on is location. In a small city, I look at a distance of about 5 miles as my maximum. (Both for initial desirability, and for long-term convenience for the nanny.)
There are loads of good question lists on mumsnet, the web in general etc.
And you could try childcare.co.uk as another site for an ad - one good thing is it lets you sort by distance.
Bad luck with the 5 - I had a day wasted recently (only 1 person, but I do longer interviews across the school run), so totally sympathise. On the upside, I had a lovely clean house by the end. ;-)