I pay my (excellent and very experienced) nanny 9.50 net p/h (contract is gross). She asked for 9 net when she started last year, we've just increased it to 9.50, planning another increase next year.
But I think we got very lucky as I think 10 net is standard. 12 net is for nanny shares AFAIK.
I think pay expected partly depends on how attractive a job it is. For example is it full time, what hours, how many DC, what ages, what will you offer by way of kitty/paid for activities/car use, do you have any special requirements that might make the job more difficult, is it a nice area with lots of activities/nannies to hang out with, etc.
When working out what you can pay, do bear in mind the additional costs of having a nanny ie kitty, activities, extra heating, providing lunch usually. In a couple of years' time nanny employers will need to pay pension contributions too (eek).
For the ad, I might put something like: "We are willing to pay 12/h gross (which would equal around 9.50/h net assuming you have a standard 1000L tax code)."
1000L tax code is the standard one and means no other job, no student loan, no rebates re pension or charity payments, etc.