I found the agencies useless ! Also the VAT rate is 21%. If you employ someone yourself you are more in control and can build up a better relationship with the nanny. If the agency is messing the nanny about you could loose the nanny through no fault of your own.
I got a nanny because my kids are very spread out in age with the older 2 being 11 and 14. There was no way they were going to a minder, I had been working around their school hours with a creche for younger children and a cleaner. But I was exhausted and the cost was not much less than a nanny. Because all of my children are now at school the nanny is really a housekeeper/nanny. She does shopping, cooking, light housework and laundry, school collection etc. Choosing the nanny was quite difficult, I interviewed some women who would have been great for the small kids but would have driven the older 2 crazy ! 6 Months in its working out well.