Hi,
I would recommend checking out the Best Bear website:
http://www.bestbear.co.uk/
Best Bear vets nanny agencies to make sure that they are really running all the checks they say they do. I looked into the site for an organisation I'm part of and was impressed with their methodology and thought they were a good resource. There are many nanny agencies out there that do a great job interviewing and running back ground checks prior to placing a nanny. There are others that don't run any checks. Best Bear has sorted them out.
They also provide some really useful information on your nanny search.
Of course, even after a recommendation, you should always make your own judgement. I've found that if I don't really click with someone at first, they probably aren't going to work well with my family.
Good luck!